Know that all major scales are essentially the same, they follow the same pattern. The only difference is that they are transposed, so the difference between C major and C# major scale is that all notes are transposed by 1 semitone.
So if you want to take the short route, you could just use C major (white keys) and have the program (fl studio) transpose it to whatever key you want in major -- or if you prefer, you can do it yourself by knowing the scales and playing them directly. Same thing with minor.. But you'll want to start with a major scale first because the vast majority of songs are in a major key.
Your friend is right, as far as music theory it all boils down to SCALES. and then you create CHORDS from the scales. So it starts with scales. There are 7 chords in a 7 note scale. Figure out how to find them. They're called the 'diatonic chords', and they usually are in roman numerals so like I, ii, iii, IV, V, vi, vii, etc. shouldn't be hard there's only about a trillion websites and youtube videos dedicated to this.
oh and don't worry about reading sheet music or anything like that, especially as a music producer that works in fl studio that should be the least of your concerns. Just worry about scales and chords, and choosing instruments, sound design, and beats.
And once you figured that out, you layer your music with the 3 fundamental building blocks of music:
- Melody (scales)
- Harmony (chords)
- Rhythm (beats)
Note that in a music program, you could have multiple instruments for each of these, and multiple tracks for each of these. but "usually" in production you start with rhythm first, add harmony, then melody. so..
- Start with a beat (look on youtube how to create a beat)
- Figure out a chord progression to layer on top of the beat. (assuming you know how to find diatonic chords of the key you are in... figure out what I, IV, V mean, etc).
- Add a melody (usually called a 'lead' which will just be the notes of the key you are in.. aka scale. sometimes instead of an instrument you can just sing... singing = melody).
p.s in between 1. and 2. people usually add a "bass-line". but the bass-line is usually just the root note of each of the chords in your chord progression.