1. Get a good photo, preferably a couple in black and white, with different moods/styles. You can generally get these cheaper if you find a college or university that are running National Diplomas in Photography and contact some of the students. Most will just charge you costs as you are offering to be a practice 'model' in exchange. So it works out good for both of you and you both get something for your portfolios.
2. Sort out your CV.
3. List yourself on as many free web directories as you can. You can also set up your own website for free. Yahoo and AOL provide free space for members and www.freeservers.com provide free space in exchange for banner ads.
4. Go to as many auditions as possible. This is the most useful advice I can give.
5. Gain experience as an extra.
6. Try to get involved in independent / fringe theatre and lo-no budget independent films. The lo-no budget films option is good as you can sometime swap skills. You be in their film in exchange for a copy of the finished product on tape or dvd and a credit. (For your showreel). Make sure you at least get expenses if there is no wage. Again it would be good to contact film students.
7. Go to an Actors' Workshops like www.actorclub.co.uk.
8. Make sure your work while you're at college is video taped. This means that if you ever get an opportunity to access and editor or video editing equipment (for example; someone who has access to an iMac may have simple editing software pre-installed ), you can make up a showreel. This is a great thing to send out to agents. Remember to enclose a stamped, self-addressed envelope, so they can return any items you send, or you will not see them again. You could invite a photography student. Many have reportage style projects and this is something again that could benefit you both.
9. When you have done all this, you can try approaching the main agencies. These are listed in film directories, like Kays, Kemps or The Knowledge. These are usually available in bigger public libraries. You may have luck in your own college library, if not you can usually get them to order one.
10. You can also send out your CV to Casting Agencies. This is especially good if you become a familiar face at auditions / casting calls, as you can get company names and contacts and look them up and write a follow up letter.
11. REMEMBER: Go to as many auditions as possible. This is the most important!