Sword Art?

[Frisky Dingo]

Which one is the first game and do I need to know anything about the series or anime before i start it?

[🦜Cloud Boy🦜]

Buy sword are Online

[Photon]

The Sword Art Online games aren't canon, meaning you don't need to watch the anime or read the light novels / manga.

The oldest SAO game available on Steam is Hollow Fragment, so maybe start there? None of them are really connected to each other so you can choose whichever one takes your liking.
Just note that some of them may not be optimised well for PC, since all except for Fatal Bullet (IIRC, not 100% sure) were meant for, and developed for console. So the PC ports may not run the best.

But no, you don't really need any prior knowledge, or need to start at a certain game

[selaCk]

MMO?

[Gian]

You don't have to watch the anime but watching it is not a bad idea. If you want to play the SAO games in the correct order you have to start with Infinity Moment, but skip directly to Hollow Fragment because the entirety of Infinity Moment's campaing is contained in Hollow Fragment.

This is the correct and complete order to play SAO games:

1. SAO: Infinity Moment
   
2. SAO: Hollow Fragment
   
3. SAO: Lost Song
   
4. SAO: Hollow Realization
   
5. Accel World Vs. Sword Art Online
   
6. SAO: Fatal Bullet

Note that all the games are avaiable on Steam except for Infinity Moment that is only avaiable on PSP.