It's super easy, you just have to hit "place bid", then it will prompt you to register if you are not a member yet.
If you get outbid, ebay will send you an email so you can go back and increase your "higgest bid" if you really want the item. just really watch your item in the last hour, b/c people will bid with like 30 seconds left and swipe your item away from you. that happend to me once it really stunk! But no I know so it hasn't happend since.
You may also be able to "buy it now" for a preset price by the seller. If you REALLY want something and "buy it now" is an option, I suggest using it, b/c it's a guarantee that you will get it!
Most sellers accept paypal, and it is a super easy way to pay for stuff! So I suggest signing up for that also. All it does is keep you credit card info and shipping info on file so check out is much quicker and for some sellers they don't want to take the risk of a check in the mail so they only accept paypal. Plus it speeds up your shipping b/c the seller receive your money quickly so they send out your item quickly.
good luck and have fun!