Online is easier than campaign...LOL. All of the starter guns are pretty easy to use, I preferred the FAMAS at first but the other AR is easy to use and a good weapon as well. Random advice: don't worry about your K-D ratio, get out and explore and learn the maps, find the hot spots and places you can get easy kills (like narrow bridges, alleys, etc, that don't take much aiming), if you die, watch the killcam to both see how the guy killed you (which is a nice help for new players, to see how better players aim and whatnot) and to find out where he is. Watch your teammates, follow them, see where they camp/snipe/run to to get into the action, also remember sometimes going the opposite way is better so you can catch some guys off guard.
Really, learning the maps and knowing where the action is and where guys are likely to be is as important as being able to aim well. If you can anticipate where enemies will be, and you will be able to once you learn the maps, you can get alot more kills than if you have great accuracy but don't know where guys are coming from...