Hi
If you feel that you want to tell them then that is great. I would advise that you tread carefully though as they are not likely to understand why you do this.
These links may help
Self harm: A guide for young people:
http://www.mind.org.uk/Information/Bookl...
Understanding self harm:
http://www.mind.org.uk/Information/Bookl...
the information is easy to read and understand.
It may help you as there are some tips on things that you can do when you feel like harming yourself.
I'd suggest that you ask your parents to read these links as well. They can then get an insight into why people do this and hopefully they will realise that it isn't something that just you do, but that others do too.
It will also show that you are asking them for help about a matter which is serious to you.
Hope this helps.