You always start out in Peltast armor, the lightest variety. It takes the least punishment, but is the fastest. Heavier armors are slower but offer better protection. When you're killed, you respawn in the default of light armor again, and you'll have to buy your custom gear all over again. Your teammates will thank you for being quick and efficient.
Some weapons require particular equipment to use.
The laser rifle can only be carried by a light armor with an energy pack. The mortar can only be carried by a heavy armor. Be sure you have the right gear for your weapon. The repair pack has a very short range. If you try to deploy stuff and get an error message, it means you're either looking too close to yourself or another object, or that you're looking too far away.