HC8H4O4K + NaOH = H2O + H4C8O4KNa

I'm given the volume of NaOH in mL (ex: 9.11 mL), and I have to convert it to moles. How can I do that conversion?

Please help by explaining if you can, I've had trouble with this problem for the whole week..

I will assume that this is a titration problem. You know the mass of the KHC8H4O4 (potssium hydrogen phthalate) and you titrated that mass with NaOH.

mols phthalate = grams/molar mass.
Since the equation is 1:1 (1 mol phthalate to 1 mol NaOH) the mols NaOH = mols phthalate
Then you want to determine M NaOH. That is M NaOH = mols NaOH/L NaOH.
In the future you should be aware that it is easier to answer a specific question than a general question. I had to guess what you were doing in order to answer your question.