How do you know what scales to use with wich chord progression?

Answer 1

Most chord progressions center around a certain chord that feels like the "home" chord. This is often the first chord in the progression, but not always. So you can start by using that chord to tell you which scale. For example, if the progression centers on an E Major, use the E Major Scale. You might also want to learn about Modes and use some of them too.

Answer 2

If you mean the I -> IV -> ii -> V7 -> I, than the question is irrelavent. I'm sure you would not ask an irrelavent question, so I'm guessing you are seeing chords as seperate entities that belong to themselves and not to the specific key which they are in. 1) determine the tonic chord: this will most likely be the first chord or chords that many Dominant 7 chords resolve to in the end (i.e. E7 going to Am VERY often would suggest A is the tonic, suggesting A Minor, wheras G7 going to C all of the time, and ending their at the end of lines would suggest C Major).
Most important, look for cadence points - points at which the song sort of has a break, like a comma or a period in a senctence. That will also tell you what scale to probably use. Cadences will end with mostly three kinds of chords - the tonic (in C Major CEG, or C Major Triad) the Dominant (in C Major, GBD and maybe F for the Dominant Major minor 7 quality), or the sixth scale degree (in C Major, ACE, or A Minor triad) which is what is called a deceptive cadence. Most tonal music is built somewhat with these cadences, so look for those. If you need to, look at some music theory text books, or maybe find some mutli-voice sheet music and find out what scale they use with the chords that are made up within the voices.

Answer 3

A chord progression (or a whole song) is usually written in one key. That key would usually indicate what scales you should use.

But music rules are made to be broken, and studying songs that you think sound good, and seeing how they did is the best thing you can do to learn.