How do you know when your ready to have sex?

This might be a pretty personal question but how do you know if your ready to have sex for the first time? I've had two ex's in my life and I never had sex with them because it didn't feel right and they always made me feel bad about it. Now I'm worried that since I'm with my current boyfriend if I ever happens were he wants to have sex I might feel bad to say no and go through with it even though I don't know if I'm ready or not. Mostly because I don't want our relationship to be how it was with my ex's. So I'm asking for the best advice on how to tell if your ready or not. Is it a physical thing? A mental thing? Or a feeling you have?