Q. I Have done so many terrible sins. How can I start doing T'shuvah?
 

A. No matter how low a person has fallen and no matter how severe the sins he or she has done, the doors of true T'shuvah are never closed and G-d's hands are always outstretched and waiting for us to return to him. The main part of T'shuvah is to STOP doing these sins. The next step is feeling bad for what we have done and asking G-d to forgive us. (But obviously if someone is not ready to stop, it doesn't help to ask G-d for forgiveness, and it doesn't help how bad they feel).

I would like to refer you to the following articles that can help you on your path to T'shuvah. May G-d be with you!

1)  Tips from www.truekabbalah.org

2) The Power of Tears

3) On-line book "The Art of T'shuvah"