I plan on watching 5 Centimeters Per Second and I would like to know if there are any prequels I should watch before I start with it.
No, there are no prequels. There are also no sequels.
Watch the anime (1 hour runtime; sometimes split into three segments, but they're meant to be watched together). Then, if you'd like, read the manga, which is more or less the same content-wise, but offers a slightly different ending, which
some might find less soul-crushing.