Unless you know the map requires you to be a pilot, or other special class, pick a rocket/mine guy from the start. When you spawn, lay some mines around your CP / spawn point.
You can lay up to 4 mines (if you pick up more mines from a driod, the 5th mine you lay will make your 1st dissapear). Pick places for your mines that are not outwardly obvious, as the enemy can shoot or grenade them to get rid of them. I personally like to stand close to a wall or entrance-way and stick them there. You can also booby-trap your droids the same way.
Lay some mines from the very start, if you die, the mines stay where you laid them... even if you choose another character class. It is also a good way to tell if the enemy is invading your base (online), because if you see your name coming up as having killed someone, and you haven't fired your weapon lately, you know someone tripped your grenade.