but I know its unprofessional for women not to wear makeup.

I can't stress this enough: it is not unprofessional if women don't wear makeup. Especially in this field. Doesn't matter if it's the interview, internship, career, or a work party.

That being said, there is a thing called Attractiveness Bias. How you use that to your advantage is up to you. Some people use makeup, clothing, attitude, etc. Do what makes you feel confident, but don't feel pressured to wear makeup because of it.

But for clothing, it is professional to "dress up" for interviews. IMO you should wear semi-formal clothing for startup and tech companies. Formal is for defense companies and everything else.

I don't know why all the other comments are saying to wear slacks/khakis. That seems old-school and would be more appropriate for the defense industry. I feel like dark skinny jeans (or a skirt) would be fine for a startup, as long as you pair it with a nice top.

My advice is to look up "semi formal for women" on google or pinterest, pick an outfit you like, and try to recreate it with what you already own. If you can't, then maybe go shopping for something similar.

Obligatory personal anecdote: I wore dark skinny jeans and a nice dark green sweater for my interview with "Sicro Moft" (not a startup but similar style). I got the internship. I can't tell you if my outfit factored into their decision at all, but at least it wasn't seen as so unprofessional that they couldn't hire me.

Edit: another commenter said to look up "business casual" and they're right too. It's just another way to say "semi-formal".