My MBTI Personality Type - MBTI Basics - Judging or Perceiving
The functions are Si, Se, Ni, Ne, Ti, Te, Fi, Fe which are introverted or extroverted versions of the two middle letters of the MBTI types and there are 8 of them. Each of the 16 MBTI types corresponds to a pair of functions, one extroverted and one introverted. In a nutshell, the J or P of your type shows which function you extrovert.
Since you list your type as xNFx, if you are xNFP then you would have Ne which is extroverted iNtuition paired with Fi which is introverted Feeling or if you are xNFJ then you would have Fe which is extroverted Feeling paired with Ni which is introverted iNtuition. That is the first step. It has nothing to do with whether you are introverted or extroverted, everyone has one introverted and one extroverted function that makes up their pair of functions which corresponds with those two middle letters.
If you are an introvert then the first or dominant function would be the introverted one (Ni if you are INFJ and Fi if you are INFP) and if you are an extrovert then your first function would be the extroverted one (Fe if you are ENFJ and Ne if you are ENFP).
This site describes what those functions are and goes into more detail: Jungian Function Theory