Hi marjanmehr,
Welcome to PC. I'm relatively new here, but I already feel like I'm part of the family, and hopefully you will soon, too. I sincerely hope you'll feel comfortable enough to share your feelings with us here; everyone is very nice and caring, no one is going to judge you, and more than likely, someone will be able to relate to what your struggling with.
You should definitely talk to your doctor. If it's interfering with your everyday activities, it's something you should address. If you're over 18, you don't have to tell your parents anything; you can get treatment without them even knowing.
What you should tell your doctor: the truth. Just tell her/him that you have been having a depressed mood recently (tell for how long), what your other symptoms are (e.g. insomnia, bodyaches, low appetite, etc.), and ask her/him what you should do. Your doctor, if she/he is worth anything, will be more than happy to help you.
I hope this has helped, and welcome to PC!!!!
Take care,