I want to get my first real chest binder, but I'm not sure what size I need. I'm really sorry, but all of these size charts are really confusing and frustrating me. I hope I don't seem stupid. If anyone could help me out, I'd really appreciate it! I m about 5'5 ft tall, weigh about 120lbs, and my cup size is 36C.

You really do need to go off of the size chart for the specific binder you want to buy. Guessing at a size based on other factors is likely to end up with the wrong size. I highly recommend GC2B, they are very comfortable and very effective. For those, take a measuring tape and wrap it around your chest, under your arms, and above your breast tissue. (while not wearing a bra, this would be the part of your upper chest before it starts to get bigger) It is important to stand relaxed and straight for this measurement.

If you are still confused, try taking some measurements, like described above, and also below your chest, and over the largest part and then tell us which binder you are considering buying?

You can figure it out with your bra size though it isn't always 100% accurate, as lots of people actually wear the wrong size.
If your band is a 36, then your underbudf measurement is roughly a 34-36 depending on mamufacturer.
If your cup is a C then that indicates a 3 inch difference, thus leaving you with 37-39 inches. If you average those measurements, you end up with 35.5-37.5 inches. (So let's say 36-38.)

Most companies (underworld for example) will want the average size. In that case I would go with 38 inches unless you'd like to measure more specifically. Other companies (I believe GC2B is one) prefer the underbust measurement so you would want to buy a 36 if you're conservatively guessing. Almost every company has instructions on how to measure properly for their products.