I highly recommend NOT starting as a sniper.
There is nothing more useless to a team than a new player sniping. Get out in the mixer, support your teammates (revives, healing, ammo, repairs). It also helps you learn the maps a lot better.
Also, NEVER snipe on rush mode if you're on the attacking team. Its pointless ( the defending team has unlimited tickets and anyone you kill will just come back in 15 seconds anyway) The only exception is if one the crates is in a destroyable building and you want to drop mortars on it.
IMO engineer is a good choice to start because you can help destroy vehicles and most of the guns are easy to control.
I actually thought that Recon was decent on Rush. Calling in mortars after a plant can be huge.
Marking enemies is a huge advantage as well. Once you get the motion mines, those can be a huge help when thrown near an M-COM station while teammates are goingto plant.And lastly, snipers tend to die a lot less. When you're attacking on Rush and a newbie dies 30 or 40 times in a match it's tough to win.
Honestly, I felt like Medic was the easiest when I was starting. I love the machine-guns and you can get a ton of points by throwing down health packs and reviving people.
But using your equipment to support your team is the fastest way to unlock stuff. Throwing down ammo packs of you are Assault, health if you're a Medic and repairing equipment if you're an Engineer. The most frustrating part for all the classes was using the starting guns just to unlock the ammo/health/repair tool.