Im in Valor, and in a clan (which helps a lot, believe me as the game is full of veterans... but dont let that put you off, you can easily catch up!)
Some things that will help you level up quicker are:
Purchasing the resus' kit and adding skill points to the med tree. This allows you to bring someone back from the dead and gives you 10XP for doing so.
Another thing is to try and follow your Squad Leaders 'Frago' target. You will see this as a flashing white box around the icon on the mini map and also get a pointer via text at the top of the screen. Everything you do around the Frago will double you points, so a kill you get 5 XP and then 5 XP again, assists 3+3... resus 10+10 and if its something to blow up you get 15+15 iirc
Finally try and keep an eye out for your happy hour... this gives you double XP for the rounds you play in that hour.
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Those not in SVER, try not to lose to much heart over the defeats.. SVER are the best out there, part due to weapons. But now the game has opened up all maps to all teams, so rather than just defending your own, you can now defend Ravens or SVERs map... this evens things up as SVERs maps are generally better to defend than attack.
Regards the DLCs I believe Escalation is more popular due to it being 32v32v32... pretty unique with all 3 teams fighting against each other and sometimes with each other to oust the other PMC from the checkpoints.
Its one of those games where you forget your KDR and get on with the team based tactics... eventually as you level up you will get positive scores etc if that floats your boat, or you may be an expert in repairing/meds/driving or all 3.
Last night for example we had 3 squads in a Platoon on Domination.... 24 players, which once you get playing is awesome fun as you all fight on a front and if the defense is decent you can get some great action, reviving, rushing in... flanking tactics to get the AA.... brilliant game.