First off, be sure you know your cycling inseam. You pants inseam is not the same things. Cycling inseam is measured by standing barefoot with feet about 8 - 10 inches apart. Then with a book, binder or similar object raise it into your crotch as much as you can. The height of the book from the floor is your cycling inseam.
But it would probably be best to go to your local shop and have them help you get the right size bike. Getting the wrong size bike would be a poor start of something that otherwise would be a lot of fun and provide a lot of fitness.