How: What and how should I tell them?

I know its long but I would really appreciate the help please. Well let me tell you the situation here first. I was living with a abusive, mentally insane EX- mother for most of my life. There was a climax in the situation in September of ‘09 to where she started calling the police on me for nothing. So I was arrested a couple of times leading up to a court case. Until that point I had almost no contact with my father my whole life, but when he heard I had gotten arrested and the cause was her he stepped in and got me lawyers and things to help me. In turn the case was dismissed and I am now living with him, my step- mother, younger half brother, and younger half sister. Before all of this happened me and my girlfriend had gotten engaged and have been planning the wedding ever since :] The wedding is still a little ways off but, how and when should I tell my dad? He will tell my step mother about it cause that’s what he does, I trust him some, but can I trust him enough to tell him this while I have only lived with him for 4 months? Should I tell him at all yet? My ex- mother and that whole other side of my family is out of the picture for good, so they will never hear a thing about anything ever again. But what do you think I should do?