I just watched youtube videos where he found the music melody notes + chords then he said that it's better if we play the song on Am instead (originally the song play on f#m). He gave the new melody notes + chords, the notes and chords changed. How to know what/where melody notes to play? I mean everytime i arranging, i just trying out every line in first 4 fret and playing the bass of the chords i found at internet. There are many version at internet of the song (one playing in C, other sites playing at G chord) i dont know which chord fit with melody notes that i found.
Melodies don't "fit" chords.
Melody - the horizontal, linear aspect of music - simultaneously provides tones that provide part of the chords of the harmony and notes that are not part of the chords. We can refer to chord tones and non-chord tones, and that sense the chord tones are the ones that "fit" the chords.
Some melodies are all chord tones, like Amazing Grace...
Other melodies use non-chord tones emphatically...
...notice that the first notes of bars 2 and 3 are non-chord tones.
The tricky part is this: conceptually the chord proper is perceived even when non-chord tones are used. So, in bar two of Moon River the chord is
Am even a beat 1 despite the melody playing the non-chord tone
To demonstrate this point omit the melody part from the accompaniment: omit the
G from the
C chords and the
Am and the
F7. When you do that you should hear the chords clearly on beat 1 of each bar. Again those chords are clear despite the non-chord tones.
Now it should be clearer that melodies don't "fit" chords, but are a contributing part of the harmony. The harmony is generated primarily from the bass and the melody harmonizes with the bass using an interplay of chord and non-chord tones.
Now back to the question of how to harmonize a melody: you don't "fit" the melody to chord tones, you provide a harmonic setting for the melody. Make sure the bass and harmony are good, and consider whether any given melody note should be treated as a chord tone or non-chord tone.
There are 2 fundamental different approaches of learning songs:
Starting with simple songs (e.g. children and baby songs, folk songs etc. only 2 chords), going on, stage and sequence, discovering that these little tunes are elements of more complicated songs.
Learning a song or a piece on a higher level, having a fix idea what you want to learn (für Elise, Moonshine Sonate, or the Piano Concerto in b minor by Tschaikovsky ...) and being fully motivated but often disappointed, when you see it’s a long way and hard work.
So to answer your question:
Why not taking a basic workshop and progressing step by step? There are lots of videos and tutorials. And later you will find out which chords are fitting to which melody.