Credit: Owen Pearson
It is important you get a frame to fit you. If you go to a good bike shop they should be able to fit a bike to you. It is not worth buying a bike that seems like a great deal but does not fit you properly.
Types of frame
Some bikes will come with both "men's" style cross-bar frames and "ladies'" step-through frames; other models may not have a step-through version but should come in a range of sizes to fit the individual. Which type you choose is a matter of personal preference.
Some bikes are designed specially for women. These tend to be smaller and have a shorter cross bar, as many women have a shorter torso.
As a rough guide to bikes with a conventional cross-bar, if you stand astride the bike the cross-bar should be two inches below your crotch. If it is another style bike then you will need to be comfortable on the bike when you have your saddle at the right height for pedalling.
Posture and riding position
When you sit on the saddle you should be able to touch the ground with your tiptoes, and when you place your foot on the pedal your leg should be almost fully stretched to reach the pedal at its lowest point. When riding you should not have to stretch uncomfortably to reach the handlebars and most of your weight should be on the saddle, not the handlebars.
As a last rule, you should test ride several bikes before you decide which one is right for you.