If you're registered, go ahead! Adam Ginsberg, eBay PowerSeller and author of How to Buy, Sell and Profit on eBay (Harper Collins) recommends buying something as soon as you register as a member of eBay. He says that a quick initial buy does two things: It familiarizes you with the eBay site navigation, and it is a fast way to increase your feedback score.
If you're not quite ready to shop and have registered as a new member, read Buy an Item. Keep in mind that you will get the best deals on eBay by doing research — for example, checking what similar items are selling for, researching what similar items have sold for, and reviewing the seller's feedback ratings.