There are two important steps to properly fitting on your bike, adjusting the seat height and finding the appropriate frame height. While the seat height is adjustable, the frame height is something that should be measured correctly when buying a bike.
The simplest way to measure if your bike frame is correctly sized is by saddling the frame. When you are saddling over the top tube you want there to be about 1 inch of space between your body and the top tube, or if you lift the bike into you, you should have about 1 inch of space between the tire and ground. if you wear cycling shoes, you should wear these when measuring your frame.
Adjusting the seat height is easiest to do by testing it out. It is best to use a bike trainer (a piece of equipment that mounts your back wheel so that you can pedal while the bike remains stationary). Otherwise find a friend to help you stay stationary while you mount your bike. After you’ve mounted your bike, put your heels on the pedals. Pedal backwards. When your leg starts to straighten, it should only have a very slight bend at the knee, you should be aiming to have close to full leg extension with a very small knee bend. If your hips rock from side to side while pedaling than your seat is too high. Adjust the seat higher or lower until you’ve reached the proper height that matches these requirements.
Adjusting the seat itself should be easy. Bikes either have a quick-release lever or a bolt fir for an Allen key. Loosen the bolt or lever, and shimmy the seat up or down to reach your proper height.
Make sure that the seat is straight and pointed forward. Tighten the seat bolt, or close the lever.
Now, hop on your bike and make sure it is comfortable and a breeze to ride.