When you start playing the StarWars mod, you will be neutral. This means that you won't be attacked by mobs or creatures if they won't be provoked.
By pressing v (it is the default button which can be changed in the controls menu) you open the "Destiny" screen where you can choose the Faction avaible to join:
Actually there are 5 Factions: Jedi, Sith, Bounty Hunters,droids, and clones, More will be added soon.
If you join a faction you won't be able to change it again during the game (but of course you can choose different factions for each Minecraft world you create), so be sure of your choice!
After that, you won't be neutral anymore and the real game will begin: you will be able to use energy, reach new levels and earn experience, unlocking powers, fight with a long list of weapons, visit new worlds, etc.
Of course, many mobs will attack you: for example droids will attack Jedi and Clonetroopers because they belong to Sith faction. Same thing for Clonetroopers, they will attack Sith and Bounty Hunters because they serve the Jedi Order.
Every faction will have its own skills and missions to do, they will have different styles of fighting and different items to use.