There are a mutitude of bikes to choose from but if you have something that is decent thats where you need to start. I started on a borrowed schwinn with dry rot tires and moved onto a mountain bike and now a road bike. I still train on the mountain to build strength and endurance, but I was lucky to get a great road
bike used. It is nice to get one that fits as well and yes they come in all colors and
sizes. Bike shops will help you with but you can estimate yourself. If you get a used one make sure to get a
tune up.