Like Merkley said, you've got a Combat Training mode on COD: Black Ops that allows you to basically get the feel of getting into the game and the unpredictability of the gameplay when versing others online. You'll notice soon that some of what the AI does is quite unrealistic, but they are there for fodder. It's more of a chance to combine different weapons, perks, attachments, grenades, etc. together to make the ultimate class for you. I suggest doing this first before heading online. One thing though, you'll need to go online to play Combat Training but it can work offline once it's registered.
Once you're ready to make the transition, you'll have to play a few rounds using default classes before you can make your own. When you can, see if you can create different classes for different scenario's/maps/game types. Best to have a variety too and trying using different guns all the time (i.e. don't have two classes with the same gun, unless you haven't unlocked anything better). The basic ones there are:
- Ninja/ghosting
- Sprinting
- Anti-Aircraft
- Sniping
Once you advance through perks and such, you'll discover what's right for you. I can't tell you what to do from here as that's not up to me, I'm not you and I don't have your playing style. However, I can tell you what not to do:
- For the love of God, don't use grenade launchers. They've made them more difficult to use and people will spam you if you use them (as they think you are spamming them by "tubing"). I'd also recommend only using launchers against air-crafts, not people (spamming issue again).
- The AK47u is the best gun in the game and can be used for all purposes. If you are getting used to the game, this is your gun. Once you gain more experience and rankings, I suggest sidelining this as a back-up just in case you are getting your ass kicked online. It's best to use other guns as some guns go great with certain attachments/perks and such (i.e. equipping "Duel Mag" with the FAMAS goes quite well, considering the ammo count and burning it does.
- Camping will get you into all sorts of trouble. If you kill a couple of people in the one spot and keep staying there, they will go after you and kill you. It's best to move spots all the time and ambush them. This works best in Black Ops due to the multiple map types.
Anything else would be common sense. So yeah, go have fun... oh, one more thing, you'll curse ALOT. You'll come up with combinations of words you never thought existed, let along you ever saying them. You will get angry with the game, but don't break stuff.