I want to simplify your question, first you have to find out which exchange has an operational license in your country, so you can use the exchange to sell or buy the bitcoin you want. But this is not talking about the safety of your bitcoins, when you are done buying and not moving to a more secure wallet as suggested on the forums, that's another question you didn't ask and maybe you should look into too.

But you also have to pay attention to this if you don't want your assets to disappear if the exchange encounters a problem. If you're talking about local exchanges that means you should look into it because we can't direct you there if we don't know where you live.