Your bike needs to fit you. A bike that is too big or too small can be dangerous. It will be more difficult for you to control your bike, more likely you will wobble as well as it being more difficult to signal.
Below is some advice to help you understand how to fit your bike, but the most important thing is that you should feel safe and comfortable.
The height of your saddle is important for the most comfortable position and safe riding style. When you sit on the saddle, both feet should reach the floor and the balls of your feet should be touching the ground. If your feet are flat on the ground the saddle is too low and needs raising, and if your toes are just touching the ground right on your tip toes, either your bike is too big for you or the saddle is too high.
The handle bars on your bike should ideally be in line with your saddle or slightly above the saddle. You should be able to comfortably reach the handlebars and your fingers should comfortably be able to cover and work the brakes. It is very important that your fingers can ‘cover’ the brake levers and you should ride with your fingers covering the levers at all times in case you need to stop suddenly.