I'm trying to make my own character sheet but I'm lost. One of the main characters is already a high elf; does that mean I can't be one?
Yes, you can still be a high elf.
Dungeons & Dragons characters aren't expected to be unique in every facet in any edition I can think of: you can have two high elves, or even a party of all high elves if that's what you all want to do.
Different races can bring new features to the group it doesn't already have access to, so that's something to consider, but wouldn't prevent you from sharing the same race as someone else.