At starting levels I'd recommend the M4A1 assault rifle and the UMP smg. I'd then ditch the M4A1 for the SCAR as soon as you unlock it, and then work on leveling up the SCAR and UMP. These should get you through the early levels till you unlock enough new guns to see what fits your style best (personally I'm running my UMP with a silencer and "range" perk, I really like that combo).
In p2p games there will always be someone with a better connection than you (unless you are host!), but I've gone on some pretty good kill streaks with the UMP and SCAR, even when I didn't have the best connection.
Also, are you playing the "support" strike package? This will allow your kills to count towards your killstreak rewards, even if you die in between.
Playing Gears 3 and MW3 on alternating nights means I ended up being good at both, but great at neither. Ugh.
