Just follow these instructions.
Choosing Kits
Step 1: If you are in game just click the paper.
Step 2: The kits that are marked in Red aren't available for you because you ddidn't buy them with your points. The kits that are marked in Green are the basic ones, so they are Free-kits.
Step 3: Left click on the kit that you choosed.
Step 4: Have a great game with your kit. :)
Buying Kits
Step 1: Right click the book in your hotbar to open the Shop menu
Step 2: Left click on the iron ignot for buying a kit.
Step 3: You can see 6 kits that kost some points.
Step 4: When you got enough points, click one of the kits that you prefer.
Step 5: Have fun with your buyed kit. :)