I applied for Luke A's series and I came very close to getting on. I don't know quite how close, but they were paying my travel expenses to come to auditions so I think it was really far. Unfortunately I lost my confidence during the later stages of auditions, so I didn't quite make the final cut.
If you want to go on it, good luck to you. Contrary to popular belief, you don't need an agent, though it quite possibly helps. They do still choose 'normal' people - Harriet works in a cafe, for example, Sarah is a law student and Joel is a Conservative councillor. If you have a personality that they find interesting, they will have time for you.
However, there are some very serious things that you need to consider before you go for it. Be aware that you would be putting yourself on television and people may not like you - you would get a lot of comments which may not be very nice from people you don't know, that can take some getting used to. You have no control over the editing. Remember that friendships may not be what they seem - what people say on television may not be what they still feel after the show. In recent years Big Brother has been known to manipulate people to achieve a desired outcome - if you're heterosexual and you get on very well with someone of the opposite sex, for example, they may try to matchmake you and create a showmance, even if you're not interested in them in that way - this happened with Danny and Sarah, and also with Jade and Nick. Think hard about the effect that it could have on your job - if you want, or already have, any sort of professional career, doing a show like Big Brother could really jeopardise your name. Don't try to use it to get a job in the media or performance industries - in the early years, people such as Brian Dowling and Jade Goody became very successful off the back of Big Brother, but these days that doesn't happen (the last person to do well in that sense was Josie Gibson, but that was five years ago and even she was the first one for about three or four years). Big Brother is no longer popular enough for the Housemates to become famous, you will most likely fade into obscurity afterwards. Even those people who manage to keep their name in the media afterwards tend to become quite annoying, such as the legends this year - people will respect you more if you go back to your life with dignity. And don't expect to become rich from it - the winner gets prize money, and the others are not bombarded with media offers. I don't know if you're male or female, but we were told that girls can occasionally get a bit of money from magazines if they are willing to take their clothes off, and guys can't generally get anything.
Having said all of that, if you have thought about it, and you're sure you want to do it, the very best of luck to you. It could easily be a really fun experience where you learn a lot about yourself and meet some lovely friends. Just bear what I have said in mind, as you don't want to regret something as big as this. I am very proud to have got to the stage of auditions that I did, and I sometimes feel sad that I didn't get into the House, but for various personal reasons I have decided not to try again. At the time it was something I really wanted, and now I am older it is no longer right for me. Much as there is a big part of me that would still like to, there are other things in my life more important than Big Brother, and the negatives now outweigh the positives for me.