Marketing is notoriously difficult to get into.... but:
If you only recently (within last year or two) graduated you can do the milkround thing - applying to everyone and seeing what happens - be imaginative and apply for a variety of companies including major ones with in house marketing eg supermarkets etc.
My university careers service supports students even after they graduate - so get in contact and get them to send you graduate jobs prospectus etc. Online go to places like www.doctorjob.com, www.prospects.ac.uk and www.hobsons.co.uk.
To get into marketing is tough, and you will probably have to start if not at the bottom but as a graduate entrant.
Faiilng that be proactive - apply speculatively to companies you are interested in working for. Make it good - show desire, passion and most importantly relevent experience.
Last chance - work your way in from the bottom if this is really the career you want. Work as a temp but only for ad agencies, get a job as assistant or administator and work your way up. Will be a paycut, but if its what you really want ...
Good Luck!