Often, people want to join a clinical trial because their treatment isn't working. Not everyone can join a trial, though. Most clinical trials have eligibility criteria. These are rules about who can participate, based on health, age, and maybe other things. For example, some trials take only people who have a particular viral load or CD4 count. Usually, you can't participate in a clinical trial if you have any infections or are using medicines that might make it hard to measure how well the test treatment is working. You also cannot participate if the test treatment might harm you.
If you do qualify for a trial and decide to participate, you should be willing to follow the guidelines of the study.