What should I do when the girl I like says she wants to be friends first?

Then, as she said, give her some time! Don't be in a hurry, if you really like her then let time to increase the concentration of emotions between you! Women love a man who pays attention to details! So in everything "go with the flow" time, from the side of the silent watch her, in her need to quietly and unobtrusively come out to help her, even if it is a small thing, as long as she needs, you can, then try to help her! When she needs a tissue, just drop by and give her one! And of course don't do this too deliberately, or she'll think you're clingy and always biting her, and that's a problem! So both to maintain a certain distance, to not let others take advantage of, not to mention the good opportunity to walk away yo!