Remote control

Dear All, I hope you are doing well. I have a question and do hope you can help me with that. I was told that another person got access to my laptop via TeamViewer. They told me that from now on whenever I turn on my laptop they will be able to see my desktop and all I am doing on it so in case I need some help they will be able to remotely help me. My question is is it possible and is it illegal that another person will see my desktop and my files without asking fro access or informing me that they have access to my laptop. Can you please explain how does it work? I mean if the person connects to my laptop will I see it somehow and can I limit their access? For example is there a way that I can hide some information or several documents or my emails and what in general another person who has access to my laptop via TeamViewer can do on my laptop?Hope to hear from you soon. Thank you in advance. B.R.