Finding the key

Hi everyone I am a new member of this site.and very happy to be here.
so,
I have a problem with scales and the chords they related with.

I know about intervals and major and natural minor scales.but I want to say the right scale when some guy says to me " hey I am gonna play am7 d e Cadd9 play on it"

I cant make any comment on this but practically I know the chords which can be use with major or mino scales are need to exist the same notes.I know all modes and I can find chords on a major scale but need to find scale when somone says me chords and

there is another porblem in c major scale chords I cant use every chord they doesnt sound well togeter.I think this is like for blues I learnd that when I use 1 4 5 chords of the key it will sound better but in major or minor I dont know ? maybe 1 , 3 5 7 ?

this explains why the blues scale is odd
http://www.guitarlessonworld.com/lessons/lesson29.htm

Diatonic Harmony... try this

You need to look at the chords, and figure out which scale they fit with. If you had a Bm, D, E progression you'd be playing a II, IV, V in A - and B Dorian would work well as it's the 2nd mode of A Major.

Also, as it's not in the other post, 7th chords follow this pattern (G for this example):

Gmaj7, Am7, Bm7, Cmaj7, D7, Em7, F#m7b5 and back to Gmaj7.

Once you get familiar with this stuff, you'll also need to know about common chord substitutions. In the example you give for a chord progression, the chords are not all the same key, so you'd need to be a little more creative...

The natural minor scale comes from the 6th degree of the major scale...