Some people say don't use the mouse, but personally I find it really hard to use a tablet for anything other than drawing. Just warm up by doing a few loose sketches first. Doing that helps you find out whereabouts things are and gets you used to pen pressure. Afterwards, try practicing smooth lines and using different pressures.
well, if you are really good traditionally drawing then it's very easy to get familiar with the tablet, took me just half an hour to get familiar with mine, after that it's just practicing, getting tips and learning tablet related functions of the software you're using.
a couple of tips: *before you actually start working on something (I mean, while you're still playing around) make sure to check the settings and tweak them until the tablet feeling matches your liking *try messing around, ask, and read about the software you are using to learn how to use tablet functions, like changing brush types, sizes, whether it has pressure sensitivity, flow sensitivity, direction and texture.
Have you unplugged your mouse yet? If you didn't do it. Use your tablet for everything - no more mouse ever again (except if you're gaming I guess - I suppose it would be difficult to play certain games with a tablet, but I don't know)