Forget J/P and learn the functions first.
The distinction between J and P is one of the most important distinctions once you learn to recognize it in real life people (like yourself for example). But maybe you should focus on other things first.

INFJ's first is Ni, right?
INFP's first is Fi, right?
No. That's only what the Myers-Briggs Type Theory states.

INFj is Fi Ne
INFp is Ni Fe
No. Not if you by Ni and Fi mean exactly the same thing as in the first quote. MBTT says that an INFJ's first function is Ni and Socionics says that an INFj's first function is , but that is irrelevant if you want to find your real type, because an INFJ and an INFj is the same type in real life. Therefore both INFJs and an INFjs have the same first function, whatever that first function happens to be and whatever you prefer to call it.

Well...
I think I'm more Fi than Ni.
But you don't know that, and you probably can't determine that for sure either. What you can do is to try to determine whether you fit the descriptions of INFJs/INFjs better or whether you fit the descriptions of INFPs/INFps better, and whether you are more similar to real life INFJs/INFjs or more similar to real life INFPs/INFps.