I am new to being a DM and our group is all mostly unexperienced. We are starting off with a one off adventure set in a gladiator style arena. Seems like a good choice for us to learn about combat and teamwork. I am looking for good monsters and fight encounters that would be challenging enough for inexperienced players but not too challenging that they won't have any chance of surviving. Look forward to hearing what you guys have to say.

Ummm, I would be careful about throwing new players into a level 10 campaign.  Even if these players are accustomed to older editions of D&D, 5th edition is significantly different.  Spellcasting, allowable actions, surprise, there's a lot of things that can make combat fairly complicated  for 10th level characters, many of whom already have ASIs and Feats. 

However, to answer your question, you probably want some fairly direct monsters for a gladiatorial event.  Think Giants, Minotaurs, Ettins, maybe even a few creatures that can fly, like the Bone Devil or Vrock (assuming your part has magic weapons).

Thanks for the tips. The idea behind a level 10 one off adventure is to let the players use higher level abilities and give them a chance to fight some unique enemies all without the pressure of worrying about long term consequences since it isn't a long running campaign, just a single sit down adventure. That being said I see your point of spells and actions being complicated for myself and the players. Do you think reducing players levels to 5 would be a good compromise, or possibly keeeping it at 10 and just giving them easier fights?

If they don't mind the combat running a bit slowly, it should not matter too much between 5th level and 10th level...unless your party includes multiple full spellcasters. Depending on the choice of spell, it might take a while to get through a Wizard or Druid's turn because of all the things 5th level spells can do, but it's largely dependent on your players being thorough in reading and understanding their own spells. If you are friends with these players, I would suggest speaking to any of them planning on full spellcasters so that you have some idea whether they play on taking spells like Animate Objects or Conjure Woodland Beings, which place potentially many temporary creatures on the map, which can really change up the "encounter math" as well as slow things down if you or the players using casters are not ready for them. 

Honestly, if you're new and they're new, then go lower.

Level 1 & 2 are too low for this sort of thing, but level 3-5 is probably a good range to aim for. I would be inclined to go for level 5, as it's a good baseline where a mono-class gets more powerful, and a multiclass has a couple of levels of each to dip in, so can make good use. There are a lot of fun level 5 builds to be made.

If you go for level 10 then you're going to have players having to look up everything they can do, you having to look up every rule they need to do it, effects flying everywhere, and some powerful spells which can just outright NOPE your fight. You also have more powerful enemies to use, which means more effects, more spells (because just hitty enemies will get stale). You also have to consider that by level 10, a character should have accumulated gold and stuff from their travels, which includes the expensive material components needed for their powerful spells. 

I anticipate a fight with 7 new players and a new DM at level 10, fighting 2-3 monsters of decent difficulty, could take several hours, and most of that will be looking things up. A single round might take 10 minutes per person if they are casting spells, and that poor barbarian who just wants to swing will be waiting for an hour before saying "I attack twice!" and then moving it along. Players will disengage, focus will be lost, and your first foray into 5e will give you all a bad impression.

You make some good points. What types of monsters/enemies would you recommend for a group of level 5's to face, whether it be multiple enemies or 1 big enemy?

With a party of 7 you definitely need multiple enemies.

In 5E the action economy is key to getting a balanced encounter - trying to keep a similar number of actions per round on each side of the combat.

Since it is an arena, you can start with one battle with creatures you think might not be too difficult, then ramp up or down based on how easy or hard the first fight was.

So prep several different encounters of wildly different levels. :)

Ok, so first thing you need to know is that CR is a guideline, and will not help much without some knowledge of what your players are running. You may have heard that the Tarrasque can be killed by a level 1 aarakocra with a magical bow; the same applies to characters. That said, it will help you to get a ballpark for the fight.

So a quick plug of numbers into the encounter calculator puts a group of 7 level 5's against five CR 3-4 monsters. But this is where you need to put in the work to work out if it's balanced.

As an example: Helmed Horrors are CR4, and an Elephant is also CR4.

If you put your players against, say, four of these, then they will perform differently - Elephants are big bags of HP with powerful attacks, and which the party can potentially talk with using talk with animals, and which the party can feasibly whale on easily because of a low AC, and they will likely deal with those 76hp on each elephant easily.

Helmed Horrors, on the other hand, can fly. They have an AC of 20, are immune to 3 spells, and have immunity to a bunch of conditions. They can't be reasoned with, and they will pose a significantly different fight than the Elephants - they are harder to hit, and have advantage on saves against spells, making them really tough to deal with. The fly speed makes them annoying for barbarians and melee fighters as they can just float upwards to avoid being attacked. And with a subtle adjustment, you can give the horrors longbows instead of longswords - and now they're flying around, shooting down at the party. Taking 60hp off each of them will be a struggle.

Now, if the party is all spellcasting, then they will not struggle to deal with flying enemies with ranged attacks. If the party is all melee (paladins, fighters, and barbarians) then they will find their options severely limited, and their abilities outranged - which will make the fight more dangerous for them.

I suggest you pick themes for each of the fights - "Undead", "Construct", "Demon" etc., and then filter the monster list on this site by that type, and by the CR range you expect to use - I'd say 2-4 is a good range to use, generally using one or two CR4's with CR3's and 2's to bolster the action economy and get in the characters ways.