Most of the time the opportunity to participate in research will be offered to you by a doctor who is studying a condition you may have. If this happens, please read more about research study participation (http://www.clinicaltrials.gov/ct2/info/understand) and consider whether this is something you want to do.
Sometimes qualified researchers make public announcements or advertisements about the availability of research projects. If you think you qualify you may answer the advertisement, but you should make sure that the study is approved by a duly constituted institutional review board.
Occasionally, researchers need to study healthy, or "normal," individuals. If you don't have a specific illness, you may qualify for this kind of study. However, even these studies have to be approved by an institutional review board and you should find out if this is true of any study you want to volunteer for.