Finding the right chords for a key: there are some excellent posts on this in Think Tank II that explain this really well, but this is a quick summary: If we think of the key of C, the notes are C D E F G A B, we can also think of these steps as numbers 1 2 3 4 5 6 7. Sometimes this is written in Roman numerals like this: I ii iii IV V vi vii(dim), this is the pattern Rev_Roach showed you and it works for every key. So, if you pick key of C and want to play an inside D chord, it will be minor because D is the 2 in the key of C.
Said another way, the ii, iii and vi are written lower case because they are minor chords. The I, IV and V are written upper case because they are major chords, so for the key of C, the inside chords are C, F and G major, D, E and A minor and B diminished. To give you another example, in the key of E, the inside chords are E major, F# minor, G# minor, A major, B major, C# minor and D# diminished.
Hope that helps.
Mama Mama many worlds I've come since I first left home.