If I say yes, what do I have to do?

What’s needed from you depends on what the trial is, so you’ll have to ask the doctor or nurse who is asking you about it. They have to tell you everything they know about it, and everything you want to know about it. You can ask as many questions as you want. And you can take someone with you too, to listen and ask questions. It’s okay to take as much time as you need to decide what’s best for you.