How does a Pap smear work? What does it test for and can the doctor tell I have had a sex from this? If I tell the doctor I have had sex what happens? And if I want birth control what does the doctor do?

Is a method used to detect potentially pre-cancerous and cancerous processes in the female reproductive system. It has nothing to do with anything else. This is a normal test when you ask your doctor for birth control pills which he prescribes to you based on your age, health and needs.

So if I get a Pap smear would the doctor be able to tell if I am sexually active

No, not really, but it would be important for your doctor to know and you will be asked. If you are going to ask about birth control, they will know it is obviously going to be happening. Your doctor can discuss various options for birth control and help you to decide what would likely work best for you.

Anything that is discussed will be kept private, even the knowledge of you already having had sex.

OP doesn't seem to be able to understand what we are telling her. I won't event post links to other questions about the same topic.