My dad had no will and his girlfriend is living in his house and has everything .. Im his only child what do i do ?

You bring a legal action against her. She has NO right to anything that is not in her name or was not purchased by her (e.g. her clothing, her jewlery, etc.) because "girlfriend" is not a legally recognized relationshp: it does not give her any property or inheritance rights.
What you need to do is first contact the county probate court and apply to them to be appointed as the "personal representative" (PR) or "administrator" (some states use one term, some use the other) of his "estate," or everything he left behind. The PR or administrator is like the executor appointed by a will: he or she is the person with legal authority to manage the estate, pay any valid bills or expenses, and distribute the assets to the people who will inherit under "intestate succession," which is the rules for who gets what when there is no will. Since your father was not married when he passed, all his money, belongings, etc. (after paying valid bills or expenses) will go to his children; if you're an only child, it will all go to you.
Then, once you have been appointed as the PR, you can bring an action in county court to "eject," or remove, the girlfriend from his house, since if she is not an the deed or title, she has no right to remain there once the person with legal authority over the house wants her gone. (But you have to do this through the courts; you can't just lock her out.) You can also, as necessary, sue her for the value of any of this things that she has taken or disposed of, and/or for a court order that she return them.
We want to stress this: if she is just his girlfriend and her name is not on any title to property or vehicles, any bank accounts, etc. then everything will go to you. The legal actions we describe are complex for a nonlawyer, but with a house, three vehiciels, guns, his furniture and electronics, any money in the bank, etc. at stake, it is well worth it to hire a lawyer to help you.