Here is a list of things you can do to help the cause of fighting for drug legalisation:
Educate people about it: write something pro drug legalisation online, in blogs, mailing lists, newsgroups, web forums; write your school report about it; publish articles about it in newspapers, tell about it to your friends, etc.
Note that you should feel free to link to this article, or even quote its text, as long as you give attribution. (See its open-content, Creative Commons, licence for more information.
Contact Your Government - write to your representatives and government, and tell them why the prohibition on drugs is harmful and not justified. If possible, vote for representatives that support the legalisation of drugs.
Don't work against the cause - if you're a drug-enforcing policeman, then switch to a different department, or quit. If you're an agent of the Drug Enforcement Administration (DEA) or a similar organisation - then quit.
Last, but not least - don't use illegal narcotics. They have numerous bad medical effects, and using them reduces your credibility as someone who can speak against their prohibition.
By acting and propagating the understanding that drugs should be legalised, we can eventually create a strong, vibrant movement against the prohibition on drugs, that can eventually abolish it completely.