Shall I try to find my Dad and how, please answer!!! ?

I'm 15 almost 16 and I haven't seen his since I was 3, the last time I saw him he was in prision, but my Mum won't tell me what for (she said drugs or burglary), basicly what happened was when we stopped talking (I think, I was young) my mum had an argument with my grandma and grandad over visiting wrights, and she told them they couldn't see me anymore, but since then I haven't seen my coision who I used to be really close to and my half sister, I really want to get back intouch, also I have an other half sister I live with and her dad left when she was two and she would feel left out, I don't know what to do, what would you do, I keep dreaming of my dad, I know that sounds sad but I can't help it lol, I also dreamt he had a son, shall I start asking my mum questions or try to let it go, help please :(