You should be able to attend the study center if you would like to participate. Also, you need to be able to understand the purpose and structure of the study without knowing all details by heart.
Pre-defined criteria determine your study participation. Inclusion criteria need to be met (e.g. you need to suffer from a certain disease, you should be 18 years or older, etc.) but exclusion criteria must not be met (e.g. you should not be alcohol dependent, pregnant, breast feeding, etc.). There are no criteria that are generally applicable to all trials, because they vary from study to study. You will be counselled about inclusion and exclusion criteria in case of your participation.