Advice on finding out about my family heritage.?

I'm from England, I'm only 15. I live with my mum and haven't seen my dad in ages. My mums family don't talk much about my dad's family and when I ask them about them it's all hush hush. I really want to know more about my family heritage but I don't know where to start. When I did speak to my dad he did mention something about our family originally coming from another country and I have alive family in another country. He didn't tell me whereabouts and I just really want to know. I can't speak to my dad's family as I don't know who they are, and I can't ask my mums family because they wont help me. Like I'm said I'm only 15 and will be 16 in about 5 months and I have this strong urge to know. Even if it turns out my family are all english I wouldn't mind (no offence) as long as I know. This is really important to me. Help??