When signing up at places like Pool Sharks you will usually have the option to either join with a basic account or a premium account. The best advice for joining is to start with signing up for a basic account just to make sure you are happy with the way the sites systems work. Then quickly (probably after only a few games) move to one of the premium pool accounts as this will give you the true benefit of what the site has to offer.
It is a common mistake on the internet that everything should and is available free. Free pool games are usually of a lower quality than what you will find at sites that have a premium pool account option. Basically you pay for what you get so if you're looking for quality, interaction with other players and more features then a premium account is the way to go.