If you've been with your girlfriend for a while, you probably know some things about her father already; however, if you don't, ask your girlfriend about some basic things about him, i.e., his line of work, whether he's serious or has a sense of humor, etc.. Finding out as much as you can about her father will show her that you are interested in her family life, especially if you two are making the transition to the serious stage.
When meeting her dad for the first time, remember that you will be judged by everything, especially your appearance matters. Going to meet her dad in shorts filled with holes or acid-washed jeans, a wife-beater and flip-flops is not a good idea. On the other hand, you don't have to wear a suit and tie either-- just be presentable. Dress smart but casual, get a fresh haircut and trim your nails. Make sure everything is clean and neat, nothing too baggy.
Most men, especially older ones, judge another man by the way he shakes hands, so give the him a firm handshake. You must also be able to carry on a conversation, so when you meet him anticipate the most difficult questions, i.e., your plans for your future, your background, your ability to take care of yourself (and his daughter) and mainly your commitment to his daughter. Avoid heavy or controversial subjects; this is not the time to air views on politics. But if you are asked for your opinion, be honest, but don't be 'Yes' man.
Although you should be considerate of your girlfriend at all times, be extra polite and considerate of her in her father's presence. Pull back her chair for her at dinner, open doors for her, etc.. Her dad should see you treating his daughter like the princess that he believes she is, not the hottie you believe her to be.
When it's time to leave, you must close the deal and leave her dad with a good impression. Remain confident, let him know that you had a great time, and give him another firm exit handshake--the kind that says " I can take care of your daughter". If your a younger couple and he says something like, “Have her home by midnight,” let him know that you will have her home earlier than that curfew.
Impressing her mom is one thing, but fathers will always have a harder time accepting any new man in their daughter's life. Her dad will always be an obstacle, but if tackled properly, you can get on his good side and stay there.