If you're 18 and in full time education, you'll get free prescriptions til your 19th bday.
Then, fill in an HC1 form. Their policy seems to be a little hit and miss however, in my 1st year at uni I got free prescriptions, in my 2nd I didn't despite being worse off!! I think it was cos my Dad was giving me more money, but the reason was that my accomodation was more expensive (house rather than halls). They only seemed to look at money coming in, not going out.
To give you an idea, I lived off about £5000 in my first year, and about £6000 in my 2nd year. This was 4 years ago so maybe the cut offs will have changed a bit but if your income is much more than that there's probably no point applying!