Am is the relative minor of C. Relative minors are always 3 half steps lower (6th) than their relative major and share the same key signature. No sharps and no flats in the case of the key of C. This formula holds true for all keys. Pick any key, count down 3 half steps and that is the relative minor. I'll pick Ab for example. Starting on Ab count down 3 half steps starting on G=1, Gb=2, and F=3. Fm is the relative minor of Ab. Ab has 4 flats Bb, Eb, Ab, and Db. Therefore, Fm has 4 flats also.
Here's a little memory tool you can use for a major scale. There are 8 notes in a major scale, do=1, re=2, mi=3, fa=4, sol=5, la=6, ti=7, do=8. There are half steps between 3 and 4 and 7 and 8, all the rest are whole steps. A whole step is two frets on the guitar and two keys on the keyboard. Once you have learned the fingering for one key (on guitar not so for the keyboards), you can use the same fingering for all 12 major keys.
I have to say that most of the charts I’ve looked at on Chordie are not correct. Every thing from the wrong keys, wrong chords, wrong voicing, and lyrics. From what I gather, most people that post charts on Chordie are guitar players. Most of the guitar players I've worked with never learn anything about theory. They feel they don't have to. Nothing could be farther from the truth. Music theory is the nuts and bolts, the building blocks, and the foundation of all things music. The best thing anybody could do is to take music theory courses at your local community college, instrumentalist or vocalist alike. It's cheep and you can learn everything you every wanted to know about theory. If you can't do that at least learn some basic keyboards.