The stable master can sell you a mount. He's got a variety of breeds for these mounts:

Draft horse 50 gp
Riding horse 75 gp
Mastiff 25 gp
Pony 30 gp
Warhorse 400 gp

Or you can get into a drunken argument with a town guard.

Viator walks forward 10ft to the edge of the bridge. Instead of crossing it, he steps off of it. He doesn't fall in. Instead he seems to walk across its surface. He walks out to the middle of the water at M6.

"What is that thing?" He asks Zephyr. He shrugs as he causes an eruption of thunder all around the creature. He uses destructive wrath to maximize the damage of a 3rd level shatter. 32 thunder damage DC 14 CON for half damage.

He summons his spiritual weapon with his other 3rd level spell slot. Just as it strikes, Zephyr flies at what seems like its face to help the divine blade land. She then flies back to Viator's shoulder.

Attack 23+1 for 11.

REMINDER: Everyone was blessed and can add 1d4 to attacks and saves.

Viator creates a blast of Thunder. The handrails on the bridge splinter and fall in the water. The humming and buzzing of angry insects intensifies as Tiny flying Mosquito-like creatures, giant by insect standards drop down out of the trees and fly up out of the shrubs near the water. Two even emerge from with the folds of the creature. Twelve in total. (Multiple Stirge will be marked as '0' if you target one it may be worth clarifying which.)

"Uh-oh, looks like someone's kicked the Stirge's nest... Literally!"

The monster is rattled by the blast (Con: 15) It seems less effected by the blast than the structure.

As Zephyr swoops and circles the beasts 'head' the Stirge's aren't able to get opportunity on the illusive Owl as it flies by.

(Note: You won't have advantage as the creature is currently trying to Dodge you manage to hit just the same.)

The bridge groans beneath the monster's weight as it attempts to avoid the bird, and fails to dodge the Spectral Greatsword that slices into it.

(Josias and Zagga)

(Thank you for choosing just the right spell to make that more interesting.)

josias is gonna use surge and run forward to the monster. (should have enough movement to him.) then attack 3 times with his attack ,extra attack and surge.(please let me know if this isnt allowed.) first attack 23  second attack 16 and surge attack 12 

   first damage 9 second damage 15  surge damage 11      

crit damage for first attack 7   reroll the 1 on my surge damage 6

and the reroll the 1 on the crit damage 1

so 19 ,12 ,13 damage for the attacks if they all hit him and they are allowed.

(do i get a extra attack for surge? i thought that extra attack was just for my regular action, not my surge.)

 (As Toren said you may attack 4 times using Action Surge, however we've got a couple other notes:

Given your mode of entry (I know I approved it, but don't intend to give you an advantage for it) you will either need to use extra movement to enter the arena in the H or I column over the wall or you can start in the start room at any position. Either way it is too far to reach in 30 feet. Klong reached his position in 60.

The creature is dodging. Using your third roll as your first disadvantage and dashing to reach him I'll need one more attack roll. If you want to hold the surge, 'til you can hit twice I'd let you given that you know you'll hit at least once.

I'll need to know your route. If you run past the bugs they will get opportunity.)

josias would start at H2 (unless i wasnt allowed to and im cool with starting at 15) and go up to H4 diagonal to i5 to be out of range of the bug beside the creature(if the creature is the four 1's which i looked and cant figure it out if it is or not im not sure that i missed the random key cause there is a 6 also that josias was trying to avoid. just incase) the rest of the way should be clear straight to the creature. 

(if im starting at 15 ill figure out the route then and still use surge and stuff to make it to the creature and attack)

disavantage attack for extra attack8

"Josias Great Lance! He's arrived just in time. Even with his help will the party be able to defeat this monster?"

(Yeah number 0-9 are all monsters. The large one '1' takes up four squares.)

Josias slides down the wall into the arena and hits the ground running. He pushes himself to reach the Mossy Mound and hacks into it with his Greatsword. Tiny barks triumphantly. As he moves to strike again the creature Shambles aside to avoid the blow throwing Josias off balance. He ends up turned around and strikes the nearby tree, practically the same thing.

Tiny heads to the Glory to visit with Daisy who pats him fondly.

(We missed Zagga, or did she miss us. I'll give her a little longer before moving on.)

Arena 8: Bog

  A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P
16  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
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14  #  :  -  -  -  -  :  :  :  :  ,  #  ,  ,  #
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13  ,  ,  ~  ~  ~  ~  #  ,  ,  :  :  #  ,  #  ,
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12  ,  ~  ~  ~  ~  ,  ,  :  :  :  ,  ,  ,  #  ,
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11  ,  ~  ~  ~  ~  ,  :  :  :  ,  ,  ,  ,  #  ,
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10  ,  ~  ~  ~  ~  ~  "  ~ 2- 3#  ,  ,  #  ,  ,
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09  ,  ~  ~  ~  ~  ~  ~4 ~ K"  ~  ~  ~  ~  ,  ,0
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08  ,  ,  ~  ~  ~  ~  ~  ~  ~  ~  ~  ~  ~  ~  ,
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07  ,  ,  #9 ,  ,  #  "  ~  "  ~  ~  ~  ~  ~  ,
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06  ,  #  ,  ,  ,  ,  :  :  :  ,  ~  ~V ~0 ~  ,
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01  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
   A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P

The brute tries to Slam Josias flat, hitting AC 13 for 10 bludgeon, and 9 for 17 bludgeon. Josias gets clobbered once, but the Mound looses its footing as planks and boards snap behind it.

The Stirges buzz around attacking the party. 2, 3, and 4 move to get a nibble on Klong, 12 for 5, 15 for 4, and 18 for 4. 5 and 8 fly after Eldin,  21 for 4, and 11 for 7. 7 and a 0 think Viator looks tasty, 24 for 7, and 14 for 4. 9 and a 0 move closer to the action. 6 and the two 0s move on Josias, 7 for 4, 15 for 5, and 25 for 5. (Piercing)

Two of the creatures latch onto Klong. Eldin fends off his attackers, Viator gains a parasite. and two attach themselves to Josias.

K:12 pierce V:5 pierce J: 12 bludgeon, 10 pierce

You may spend an action to remove the blood suckers from yourself or an ally this assures success. You may attack them if they are attached to someone without penalty. 

If you are medium you can pass through their space (and vice versa) as difficult terrain. If you are small you can pass through the brute's space (and vice versa) as difficult terrain.

(Everyone is up.)

(If I try to run, will the stringes get Opportunity?)

(Not if you Disengage.)

Eldin says, "Get off my back!"

He casts Word of Radiance to hit the two stringes near him. Damage: 8 CON 12

If the stringes are still up, he will disengage and move to 4D again. If they're dead, still move to 4D and use stealth: 19

josias is gonna use his two attacks on the insects. first attack 12    first damage 15    second attack 10   second damage 13  (if either of the attacks kill or crits the bugs josias is gonna use the one part of the great weapon master to cleave thru to the next one with the bonus action to attack. so he can get in a 1 on 1 fight with swampthing.)

(annnnnddd a swing and amish i guess)

 Eldin radiates fiendish power. (Con: 5: 15, 7) One is obliterated. The other remains.

Josias swipes at his new very close friends. But finds it's difficult to fight house flies with a Greatsword. (Not any more difficult than other tools.) 

(Klong, Viator, Zagga)

Klong, having observed the thunderous boom from Viator earlier scratches his chin in between the stirges latched on to the sides of his face. He turns towards the gigantic plant, takes a step forward and says "Klong can make go boom too!" with a loud bang on his drum he casts a similar spell. When he hears the familiar laughing fighters voice he smiles, "Hya friend! Hit him good for Klong and you get glory!".

Movement: I-9 and into flag range (I thought you said stirges didn´t get opportunity attacks earlier?)

Action: Cast Thunderwave (3rd level) on the area right in front of him Dmg: 16  Save: Con 14 (This should be able to get the shambler and stirge number 0 at its side?)

Bonus Action: Inspire Josias (1d8 ! Glad to have you here)

(edited due to failed roll)

(Nothing gets opportunity against an Owl, that's why he picked it.)

Klong steps onto the bridge and releases a jarring wave of thunder. The Stirge in the blast is squashed. (Con 1: 11) the Mound is pushed off the structure by the blast, and it collapses under Klong's feet. Just as he was welcoming Josias to the fight. Klong falls into the river, and manages to splutter out his words just the same, inspiring.

Josias is cured of a parasite.

(Viator, Zagga)

(Klong inspires greatness in others by being a total jackass himself. They think to themselves "Wow, i gotta do better than that guy!)"

Concentration check for bless 4 vs DC 10.

Viator removes the stirge from himself and makes his spiritual weapon try to cut it down.

Zephyr helps by trying to distract the parasite.

Attack 11 for 13 damage.

EDIT: Forgot to roll bless for the attack 4.

EDIT of the edit: If the DM lets me, Viator will instead of using his action to remove the stirge, he will use his reaction to shock it for 8 damage DC 14 DEX save for half. His spiritual weapon will attack the large creature instead and Viator will attack the other stirge with his battleaxe causing crack of thunder as he does with booming blade. Attack 13+2 for 4+3. It will take 15 more damage if the creature willingly moves before Viator's next turn.

Initiative 14

(This is my first time playing dnd, so I'm not entirely sure what I need to do.)

lol it's your turn. You can move make an action and make a bonus action unless you have something that lets you do otherwise