If you start from scratch, you may need to stick with teammates. You WILL also die from Saboteur's disguise, Marksman's cloaking, Medics' revival. Keep your cool, observe and learn the maps.
After a while, you should be able to sense a disguise (Your real comrades should not walk/run towards you !). You'll also know the safer routes away from sniper fire. Etc.
The game plays like a mini MAG. It could be harsh like MAG, but because you have very special abilities, you may be able to cut through all the defenses and achieve the objectives even alone. In MAG, you'll need teammates' help.
Also beware of proximity mines, key areas are always booby trapped. Either wait for your teammates, throw a 'nade to trigger the explosion, or fire a rocket. The game is half gun fighting, and half using your smarts.
Sometimes, you should give up the current objective, and prep for the next one.
Sometimes, you need to risk it. Sometimes you have no choice but to do all the grunt work like grabbing spawns and repair crates.
The biggest problem about the game is the match making. If the teams are uneven, quit and find another room.
Edit: The easiest class to start with should be the Medic. As you level up, you gain auto-heal, a droid gunner to help pinpoint and shoot enemies, plus revival/second chance.
The Engineer is under appreciated. It is the best defense in the game *when near an ammo crate* because of turret, heavy MG, unlimited mines and ammo. Your enemy will target the crate but as an Engineer, you can repair it quickly.