How do I say no to my bf?

"No Means No" Firmly but compassionately tell him that you are not interested in a sexual relationship right now. If he presses, tell him if he respects you and truly cares he will not push it. If he continues then you may need to re-evaluate his commitment to you. Good luck, relationships can be difficult but also very rewarding.
Agree with Dr. Woods. Have enough love and respect for yourself that you are willing to assert what is and isn't alright with you.