Dudley has a good point, to a point (sorry, couldn't resist). To elaborate on Dudley's message, improperly adjusted bearings can ruin expensive parts - hubs, headset even the frame in extreme cases. They are not to be taken lightly.

Don't jump into bearings unless you feel CERTAIN you understand what is involved both as to the specifics of the job at hand and appropriate use of wrenches. I read, re-read, and re-re-read at least 5 different descriptions of how to do hub bearings and felt like I had enough understanding of bearings in general, familiarity with using wrenches and the principles of lock nuts to give it a go. However, I did OJT fixing up a $15 Salvation Army Schwinn and felt certain I was doing it right before I tried it on a good bike. Pulling them apart, cleaning, regreasing, and reassembling are not that hard. The critical thing is proper adjustment - not too tight not too loose.

Only time will tell and luckily none of my bikes are expensive enough to present a major financial risk. On the other hand by doing it myself, I will do it more often.

I highly recommend a cheap old 10-speed for a learning laboratory.
Regards,
Raymond