Get yourself a bike with a step-through frame (a girl's bike).

If possible, get one with an upright riding position like an old three-speed or maybe even a single speed with coaster brakes.

Wider tires will also give you a more stable ride.

Hook up with a friend who knows something about bicycles and can adjust the seat and handlebars to fit you correctly.

Ride that around for awhile and your confidence and skill will improve.