What form should I use?

If your father entered the US on a visa and you are a USC citizen over the age of 21, in theory he could adjust status to permenent resident without departing the US.  There are a serioes of facts that come into play in making the determination to do this, including whether he has any prior record with criminal or immigration authorities. The forms you will need may vary according to the case, but the petition is the I-130 and the adjustment application is I-485.  You will need to look into other forms, such as the affidavit of support.  If you need assistance with the processing of the case, we can assist you.