I moved this here since it was basically asking for personal advice, and was more of a 'practical' nature than of an 'aesthetic' nature.
Now, I am only left to wonder with the little detail that has been provided. However, since the question can be assumed to have been asked on top of a background of knowledge and information that fairly concludes that you are, in fact, in a position to marry the girl who you love, I would say that you first have to make sure you have a solid plan of action all laid out and ready to explain to the parents.
By that I mean that once you have gotten married, then what? Where are you going to live? How are you going to support the future family? How do your own parents feel about it? As far as bringing up the topic to her mom and dad, you of course firstly have to bring it up with her. If she has not first given a full-hearted 'YES,' there may not be much chance of convincing her parents--but then again, I know nothing about them either. . . so.
Regarding where to propose, I would suggest a place of nature for the actual asking--a park, a nice 'scenic' spot by a river, the beach, etc. It might be good to do so after having had a nice meal. The rest is up to you and your likings. I live way up here in the mountains, and so can't help much more than that.
I do wish you the best, and hope that all works out in your and her favor--whatever that may turn out to be. MM