Buying or selling? If buying:
don't just browse - wait until you need something, then go look for it on ebay
don't buy if you don't know what the postage will be (you can send them a "question to seller" asking what p&p will be)
work out how much you want to spend, take the p&p off that, and bid that much - don't go back and look at the item, don't increase your bid!
get an email account on lycos or yahoo or somewhere and use it for ebay (so if your email address gets out to someone who shouldn't have it or you start getting lots of spam, you can scrap it and get a new email)
be nice and friendly in your emails and give sellers the benefit of the doubt if something arrives late / with wrong postage / not as described etc - most people are selling in good faith and will give refunds if you are nice to them!
watch out for clothes - different people have different ideas of what is "good condition"
use the "advanced search" and search for "completed items" to find similar items that have sold before - you can see how often the thing comes up for sale and what it sells for - so you know what to bid and whether to let this one go and wait for the next one
is this the kind of thing you wanted?