Your sewing machine will have a bobbin winder built in to it. How it works of course depends on your sewing machine. You will look at your sewing machine manual for instructions.
Although your sewing machine will have a built in winder, a free standing tool is a good idea. It's not absolutely necessary, but as you become more familiar with sewing, you will want to make investments in tools that will make your sewing experience pleasant and without interruption.
There are portable winders that range from approximately $20.00 to $100.00 dollars that are powered by batteries or electricity. These little machines are separate from your sewing machine. A portable winder can come in very handy and here is why.
A portable tool is handy because you don't have to stop, un-thread, and re-thread your sewing machine to fill a bobbin. It sounds like a small inconvenience, but for those of you that are new to the sewing craft, small inconveniences, turn into big ones quickly.
Below are some examples of what they look like, who makes them and how they work.
If I am working with fabric seams that are different colors. Like the photo below. In this case yellow on the bottom and red on the top.
I like to have yellow thread on the bottom and red top thread. Instead of having to stop, take the thread off of my machine, and spin a bobbin with yellow thread, then re-thread the machine.
See this video on how to wind a bobbin using a portable winder.