Clinical trials as described by Wikipedia are a set of procedures in medical research and drug development that are conducted to allow safety (or more specifically, information about adverse drug reactions and adverse effects of other treatments) and efficacy data to be collected for health interventions (e.g., drugs, diagnostics, devices, therapy protocols).
All that jargon basically means that governmental organizations, pharmaceutical or biotechnology companies are always looking for guinea pigs to test their drugs on and are willing to pay large some of money to make that happen and avoid lawsuits. So if you really need cash and don't mind donating your health in exchange, all you need to do is go on the internet and search for clinical trial in your area.
Ways to locate clinical trials:
Internet / Search engines
Radio / Television
My advice to you is:
Do not overdo it (1 trial at the time)
Do not make it a full time job as you will be slowly killing yourself.
Speak to your doctor before enrolling in one as you will be exposed to unknown risks