What kind of brake pad is best for my road bike?

I just started biking this year, and have noticed my brake pads are wearing down, and I should replace them soon.

Im wondering what is best, and if there are different styles that I should be aware of so that they fit my bike!!

I have a GT series 4, with I think shimano breaks…but Im not sure what brake pads will fit it!

Im thinking about buying on ebay, since its much cheaper, but im afraid I will buy te wrong type!!

I live in Seattle weather, so pretty rainy!