Cycling > Wheels
What is a bike without wheels? Not a bike. Wheels are the most import part of the bike. Without them you would not be able to ride it. If you are looking for a lighter bike the best way to save on weight is the wheels. The lighter the wheel the better the acceleration and you will not get as tired as fast.
Bike wheels come in different sizes and to determine what size would be best for you is based off of how tall you are. If you are a child between the height of 3’8 and 4’5 the wheel size should be between 18 and 20 inches. If you are 4’5 to 5ft the wheel should be around 24inch. Any taller than 5ft they should have a wheel that is 26in.
When looking for bike it is important to check and see if the frame is only built for one type of wheel, or can the wheel be changed out for a different size one. If you are done growing this is not the biggest thing to worry about but it is always nice to have that option especially if you still are growing and plan to keep the bike for a long time.