The class I am going to recommend you use if you are new to Call of Duty: Black Ops, AK-47 with the extended mags, Ghost, Sleight of Hand and Second Chance. Ghost will keep you off the enemy team's radar when they have a spy plane out. Sleight of Hand will help you reload faster and if pro, will let you aim down sights faster. In Second Chance, especially if you are playing with a team, can help you get revived in those sticky situations where you get put down, save yourself with your pistol and then are laying there on your ass.
We also recommend you use the claymore. Since you are not the greatest player in the world, you'll probably end up finding a little building and sitting up in a window. This will allow you to put the claymore behind you at the staircase or the ladder entrance to that building which can protect you from behind. If it doesn't kill the enemy, it'll at least warn you that they're there because you'll hear the explosion.
I recommend that you also buy a headset. A cheap headset. You don't need to buy an expensive headset, but cheap headsets are really useful when playing Call of Duty: Black Ops. Sound in Call of Duty is almost more important than the visuals, and it will really help keep you alive. Footsteps from players not wearing ninja are very loud and can help you turn around and turn on people who are sneaking up behind you going in for the kill. You can also hear reloads very loud in Call of Duty: Black Ops, so be careful when you're reloading. Only reload if completely necessary.
As you can see, Second Chance helped save my life there and now I can sit here and if I was on a team, wait for someone to come revive me. I recommend the AK-47 with the extended mags because extended mags are very helpful in the AK. It's sometimes a little slow fire rate and its inaccuracy can hurt at long range, but with the extended mags, you have more bullets to get the job done.
I recommend using the frag at all times because the frag can roll to places that it needs to get to in case your throw is a little bit off target and it takes a little bit longer to explode.
Anyways, guys, this has been Patriots for RealGamersTV. Hope this helps you and go out there and play Call of Duty: Black Ops today. You can only get better.