Father gets murdered by mother and mothers boyfriend

The main character is a teenager who has a job. He meets a girl there who ends up being his girlfriend later in the book. The father gets murdered by this teens mother and her boyfriend, the motive was that the fathers rich friend died and the will of the dead friend had the father as the only person on it, the mother killed the father thinking her name was on the fathers will so she thought she would get the money. I also remember the teen sneaks into some rich guys house but i can't remember why.