Greetings maggot, I am Corporal Mathias, and don't you dare question my rank! I can see the insubordinate look in your Eyes.
First of all get acquainted with the three potential ways you can go!
Boris, the Brawler/Commando:
Now it's pretty standard, you are armed with a machine gun (LMB) and a nitrogen cooler, use the Nitrogen Cooler (RMB) to fire your machine gun at absurd rates of fire! Later on you can gain an upgrade which causes the Rapid Fire mode as I call it, to LAUNCH MISSILES. But first and foremost you must SURVIVE to get there soldier! You will fall down repeatedly, you will be blown to smithereens, but you will get the hell back up. And if that's not possible, your next reincarnation (Brought quickly here via cloning) will get the hell up. The Brawler focuses on heavy hit and running, and is the obvious choice, as you will not be slowed down as much when moving and firing. But putting some points in the Commando tree has it's merits, gaining health as your enemy's bleed to death is always nice, it revolves more around criticals and other such utility stuff. I myself prefer guaranteed damage via a barrage of rockets and bullets.

Gidrun, the Assault/Droidmancer:
Don't let her looks fool you, this lady is a cold hard killer, packing a shotgun with a mean punch aswell as a modernized sword (Spacebar to switch between the two). She can be harder to learn then Boris, as it may take some of you time to realize when it's a better idea to fire the shotgun then run at a punk with a sword, she's also capable of dashing quickly to close the distance. Wherein you can then unload a barrage of shells or slice them to pieces. The Assault Spec focuses on increasing the potential of Dash and also has some nice perks for your shotgun, whereas the Droidmancer focuses entirely on your Gundroid companions (Yes, she has two. Quit your whining, Tina and Boris have their own merits in combat). Now personally I rely more on myself then others, especially robots, but to each their own.

Tina, the Bounty Hunter/Gunslinger:
Others may dislike being surrounded, Tina's the exception, she simply readies her two machine-pistols and strafes about the field. Wasting anything around her, remember that she moves towards where your mouse is pointed maggot while she is strafing! Bounty Hunter focuses on criticals and keeping her ammo count up (Because unlike Boris, she can't fire her gun when out of rounds, she does regen ammo aswell as energy though.). While Gunslinger focuses on strafing and fancy gunplay.