For most people, getting their vision checked is part of a standard eye exam. This allows an eye doctor to find out if you need vision correction, like glasses or contacts, and if necessary, lets them run secondary tests to figure out exactly what level of correction you need. If you've never had glasses or contacts before but think you might need them, this is what you can expect.
The first thing that your eye doctor will do is to have you look at a standard eye chart. This is a chart containing multiple letters and numbers in varying sizes. All you have to do is read what you see. If there are discrepancies between what's written and what you say, your eye doctor will likely make the assumption that you have a problem with your vision and may need vision correction. Try to be honest and not guess — if you can't see something clearly, just say so.
The next thing your eye doctor will do is to run a vision correction diagnostic using specialized equipment. You'll sit down and look through a viewfinder that will show you a small image. It will appear to be out of focus. The machine will assess your vision by looking at the angle of your cornea, which can warp light as it enters and make your vision appear blurry. Once it's made an assessment, the image will shift and should become more visible. You don't need to input or respond to anything at this point — just let the machine do its work.
Your eye doctor has one more job to do before they can set you up with a vision prescription for glasses or contacts. They need to make sure that the machine's assessment was correct.
To do this, they'll bring you back into the room and will have you sit behind another piece of equipment. This equipment is loaded with multiple lenses of differing strengths and magnification levels. They'll flip to the approximated vision correction level that was suggested by the test earlier and will have you look through it and read the vision test again. You should be able to see it much more clearly at this point.
From there, they may have you look through other similar lenses to ensure that you're receiving the best vision correction possible. If something looks better or worse to you, just vocalize that to your doctor.