If you find you are unable to tell your parents and unable to find a place to create as your altar, then i would advise you use this time in your life to seriously strengthen your basics and energy control.
If you make a place an altar, it becomes sacred grounds and it will be defiled if you have other people come around it without the proper respect or touching it without know what it is. Their energy will be disrupting the energy flow of your altar, defiling the area and making it useless. You can always cleanse it before every use, but that also means it wont have the time to gather your own magickal energy so it still wont be very good. If you feel you need one at this time, then perhaps you could consider making an altar in your back yard or in a near by woods. Those would be great places to make altars anyway. If you decide to do that, just make sure to find a good place.
If you find you are unable to find a good place to make an altar at this time, then i would greatly advise you take this time to strengthen your core basic skills and use spells that don't need one for now. Your basics are what define your ability to use magick so as long as your properly strengthening those areas, you are becoming more capable of using magick. All of these areas are things you can do pretty much anywhere. It does help to have your own space to go to when doing this, but it is also something that can be done fine in other environments.