Help please

So I have been talking to this guy for a while, we met and kissed blah blah no other romantic or sexual relations im young ok chill. He has a girlfriend but its not me. I knew this and I still kissed him and everything, what do I do? I want to be with him, can I trust him if we get together? And how do I get him to pick me and not her? Do I wait or sabotage the relationship????? Please help.