It's really simple to join an online poker room. You could use an online payment service such as Neteller or simply use your credit card to buy chips just like you would buy a cheap flight from easyjet or buy a book at Amazon. First rule is "you dont need to give them any money to get a basic feel of a online poker room". You can play on free games without handing over any credit card details. You should try playing the free games for a minimum of an hour to get the feel of the table, find out what buttons do what, how fast the action goes, just make sure you are familiarised with the ins and outs of the online cardroom before you spend your hard earned money. The free tables will not teach you to play winning poker but are good for learning the basics of the site. You should not stay on these free tables for too long as the players you are playing against have nothing to lose, as do you. This creates an artificial environment which will not help your game progress.
Spend some time thinking up a creative login name. Some online poker players want a powerful alias, some prefer a memorable name. Others want to stay inconspicuous. Your screen name will be the first bit of "table image" that you give to other players. Try to choose a screen name that represents your personality or the way you want other players to percieve you.