How to say no to other people?

Are you a gym rat or a couch potato? I want to be in the middle.

When I want to do play sports, go to the gym to do yoga, running, or go to swimming, or climb mountain. If I would want to relax and I just want to lay lie on the sofa, drinking tea, watching TV or reading a novel.

I did yoga and running last night. After I finished, I was waiting for the lift to went go home. A beautiful girl who was still waiting smiled and said hi to me. Then we talked. She is very active and asked me many questions. Actually , I don't didn't want to answer some because they are were personal. Although we are members in of the same gym but she is still a stranger to me and we just met first time. But Finally, I answered all her questions politely. This made me feel wired.

What's the matter with me? If I don't want answer her questions, I can say no. Why did I say yes? I can't follow my heart because for some reasons, like to be polite or other reasons. I can't don't have the ability to say no to other people. This is also not a mature performance. How to say no when I don't want to do in my mind?

I understand how it feels, I think its quite common for many people. I have also experienced those kind of situations alot. Its takes alot of courage to say no to people at times. Well, I think sometimes saying yes is sign of respect when you talk to someone. (doesn't mean you can't follow your heart).