Freeroll entry requirements vary across the poker sites that run poker freerolls and also vary greatly depending on the level of prize money available. There are two specific reasons for poker sites to run freerolls:
1. To get new players to come and play poker at their site.
Nobody gives money away for nothing, although the free rolls do have cash prizes and you can win real money at them, there is a reason for that. You need to sign up at the poker site to play the freeroll so they now have you as a member of their site. The hope is that you will go on to play poker for real money (which is a good thing by the way, it is the only way you can possibly become a better poker player).
2. To reward or incentivise existing players on their site.
When you are a money player at a poker site, the poker freeroll entry requirements are a bit different. The game has now changed to being a reward for playing on the site and instead of playing in the $10 - $50 prize money range, you could now find yourself with entry to a members only freeroll with cash prizes in the $100 - $500 or more bracket.
The freeroll entry process itself is generally a very simple thing. First of all you will need to sign up to the poker site as a member. This is a requirement across the board at all sites that run poker freerolls, there is no way round it. You need to sign up in order to get a player ID name to allow you to register for the tourneys.
So sign up as a member of the poker site and get your player name. Next you need to download the poker client - a piece of software that sits on your hard drive and is the interface that allows you to play poker in real time. Once you've done that, just go to the tournament section and look for the freerolls.