First and most importantly: you should be dating someone you aren’t scared to talk to, and who doesn’t make you feel like expressing your wants and needs and boundaries will make him leave. Anyone who makes you feel like their affection and desire for you comes with strings is not anyone you want to have in your life. Period. End of story. You can do better.
Tell him you’re not ready. If he breaks up with you, he does not care about you and you are significantly better off without him. If he continues to pressure you, please know that that is coercive, and wrong. If he says anything like “Well, if you don’t sleep with me I’ll break up with you,” that is coercive, and wrong. If he says he’ll tell people that you did anyway, or spread any other rumors, that is coercive, and wrong.
Coercive rape is sexual intercourse that comes as a result of pressure and manipulation — in this case, emotional manipulation. Your boyfriend, though he may not realize it because our society is a horrifying ocean of gendered violence in which we are all struggling not to drown while simultaneously pretending we’re enjoying a picnic on dry land, is acting like a sexual predator.
Do not do anything you’re not comfortable with.
First of all, it just won’t be fun; if you’re not ready, it won’t feel good. The entire point of having sex is to feel good. (By the way, it doesn’t have to hurt the first time. That is a myth perpetuated by the misogynistic idea that women are less interested in sex than men, and it’s a nice little loophole for men to get out of making sure you are comfortable and happy before having sex. Sometimes it will regardless of how turned on you are — sex is nerve-wracking the first time.)
Second of all, you should not put your needs behind someone else’s out of fear. Someone who is asking you to is someone you need to excise cleanly and neatly out of your life, forever. You deserve to be treated well and with respect. This is hardly respectful.
My tips are that you tell him you are not ready — just like that, no wishy-washy “I don’t think I’m ready” or “I’m not sure,” just “I’m not ready for this” — and if he reacts in any other fashion than “perfectly fine,” dump him. If he spreads rumors about you, shrug when someone confronts you with them and say, “That isn’t true, but it’s cute that he thinks I care what he says about me.” Then tell people he has a tiny penis. And mace him if he comes near you again.