Downloaded this out of sheer boredom, just about to jump in on Miller. Not sure what class to go with or faction tbh.
Normally regarding faction I'd say pick the one you like the look of or like the weapons/vehicles of. Make a character go to a warpgate and use the terminal in the middle of the building (with a planet icon on it) to change continent, pick VR training (button at the bottom) and try out all the weapons, vehicles etc in VR for each faction.
On Miller I'd say Vanu Sovereignty need more players most of all. If you don't like their spandex uniforms and lasers then pick New Conglomerate, Terran Republic have more than enough people as it is.
In terms of class you're not stuck playing one, over time you'll get enough certs to loadout all of them. I'd say to start with for infantry combat medic or heavy assault would be good to start with. As a medic you'll have an assault rifle which are generally good all round guns, you'll also be useful to your team healing and reviving people which will help you get certs early on. Heavy assault would be better if you just want to kill stuff, you'll start with an LMG and you'll also have an overshield you can turn on in fights to absorb damage so you might last a bit longer and a rocket launcher to use against MAX units and vehicles.
If vehicle combat is what interests you most then engineer will be what you want as they have the repair tool but be warned vehicles in general require a large amount of certs to be spent equipping them before they truly become effective. What I mean is, I'm max level but you can still shoot me and kill me with your gun at level one. If I have a fully equipped tank vs your stock newbie tank I have a greater level of advantage.
Certs = Certification points, you earn 1 for every 250xp you get. You get xp for virtually anything you do in game from kills, assists, repairs, heals, capturing bases etc.
Anyway, any questions just ask here, we have people of all factions and on several servers.