How to use scales

I want to use the scales but i am supose to change the chord for part of a scale or ...

the way i would go about doing it, is to first come up with a chord progression...so lets say your progression is Em-Gm-Am. the next thing you want to do is find "home base". basically the chord/key that your song is in...in a standard rock song, you determine the key using the first shord you play...in this case Em. however, another way to do this is to determine which chord your song resonates off of...for instance, lets say you play Fm-Em-Gm-Am. but you are sliding into the E minor chord from the Fm something like this..
E:1/0---0---3
B:1/0---0---3
G:1/0---0---3
D:3/2---2---5
A:3/2---2---5
E:1/0-0-0-0-3
you arent really hanging on the F chord...you are simply using as a lead up to the E chord...you could say that the song is in the key of E...
basically you need to figure out what is the chord that is main tone of the progression.

then when I have the chord that is the main tone in the progression that is the scale that I need to use for the song?

Well, say if you have Em - the easiest option would be the E minor scale, or E minor pentatonic.

Although... the first chord isn't *always* the 'main chord'/key - so if I were you I'd read a few lessons on the subject of chord construction, chords in a key etc etc.

Hi There. As Doz said, the first chord isn't always indicative of the key. When you are trying to find the key for a chord progression, you basically have to figure out which key contains the greatest number of the chords you have in your progression, and that will be a pointer to the right key.

If you look at the pic I attached of the major keys, you can see that Em is included in alot of major keys - it could be C, G, D or Gflat major. The more chords you have in a progression, the easier key analysis gets (within reason).

For example... if you had a progression that was Em, Am, F - you'd probably be in the key of C, because the key of C is the only key that has all those chords in it.

(although you could also be in the key of Gflat, since in rock the diminished chords are sometimes played as if they are not diminished... but thats an unlikely key :)

Sorry... figuring out the posting image thing... Also, disregard the Gflat comment - its Ebminor in that key.