I've read it over a few times and I'm still not sure how to interpret it. For the extra attack on you first turn do you able the normal damage plus the additional D8. Or simply roll the D8 on a hit. I'll include the paragraph below for ease of reference. 

"At the start of your first turn of each combat, your walking speed increases by 10 feet, which lasts until the end of that turn. If you take the Attack action on that turn, you can make one additional weapon attack as part of that action. If that attack hits, the target takes an extra 1d8 damage of the weapon’s damage type."

You get 1 free extra attack (single attack) when/if you take the attack action on the first (and only the first) turn of a combat. That attack, and only that attack, if it hits, gets an extra 1d8 damage. 

Where I'm unsure is say I attack with my Short Sword as an example. For damage would I do the 1D6+M+1D8, or just the 1D8 without the 1D6+M.

Quote from Hadhod_Harpstring >>

Where I'm unsure is say I attack with my Short Sword as an example. For damage would I do the 1D6+M+1D8, or just the 1D8 without the 1D6+M.

 You nailed it! Shortsword, plus ability modifier, plus the 1d8. It's a big hit.

Thanks for the help. I thought it must work that way, but wasn't sure.

for future  reference Anything that says extra means in addition of other effects

It says "extra d8"  meaning in addition to the normal damage. I use a hand crossbow and took a level of rogue initially giving me extra skills and the extra d6.