I think the best tip for any FPS is to just play more to get better. The equipment you use is really personal preference. I'm not sure if Black Ops has equipment that is clearly better, but even so it still might not matter. As an example, I run around in BFBC2 with just a pistol most of the time and I do fine.
Some tactics (I use in Battlefield games):
I try to not fire at everyone I see, so I can evaluate the situation and figure out if giving my position away is worth it or if I should move around to intercept my target another way. Another thing is I use grenades to "smoke" people out. If I know someones at the side of one wall, for example, I will throw a grenade on that side and force them to run around the other side where I'll be waiting to shoot them. If they don't notice the grenade they simply blow up.
