Be careful to use all the same particles with phrasal verbs. You need the whole phrase "stick up for" to have this meaning. It means to defend yourself. If you just said "stick up" it would mean to rob someone.

“Stick up for yourself” means that you defend yourself after being harmed, attacked, wrongly accused of something. You can stick up for yourself by communicating your opinion.

To stick up in that sentence means to defend yourself