You got to pick up 4 cards that make up a hand, well you already know that by know but the following would be the top slots, the below ones are called traits and it is not very easy to get them and they do cost handful of Credits each, before you unlock them you should defiantly have some info about these traits.
Check out the best cards built inÂ Star Wars Battlefront
The Explosive Setup
The Anti Vehicle
The Long Range Setup