The best starting place is the major scale, or the first (Ionian) mode of the diatonic scales. Most people don't realize that there are 1-3-5 chords associated with every note in the scale, built relative to the note in the scale. So, if you start with C major (easy, since their are no sharps or flats), you get the following chords:
C - Dm - Em - F - G(7) - Am - Bdim
The seventh chord is a half-diminished (full diminished is usually expressed as a dim7), which means take a standard minor chord and flat the fifth. The fifth chord is usually a seventh chord, although in the rock indium, it is usually substituted for a standard major chord.
So, if you hear someone say, "Oh, that is a one, five, minor six, four tune," they mean the first, fifth, and sixth chord of the major scale, which in this example would be C - G - Am - F.
The great thing about diatonic modes is that you only have to learn one scale (theory, not fretboard pattern). All six other modes simply change the note/chord you start on. So, when you hear that a song is in Am, it usually means that it is natural minor, or the sixth (Aeolian ) mode of C Ionian:
Am - Bdim - C - Dm - Em - F - G(7)
So, the big jump start is the minor verse, major chorus style of songwriting that you hear all over the place. Make your tonal center Am in the verses, giving the song a darker sound, and then make the choruses in C, giving things a happier sound. This relationship is often called "relative major" or "relative" minor. So, if someone tell you that the song is in G, with verses in the relative minor, they mean the verses have an Em tonal center.
Of course, there are no rules in songwriting. A learned a bunch of theory, but I don't think theory when I write riffs; I simply go with what sounds good, and then hunt for a scale to match it to. However, the theory is important, since it gets in your head and effects how you think. If I'm 80% of the way there, and looking for the next change, I analyze what I have, figure out if it fits one of the modes, and then start trying out the other chord changes made available by that mode. I can also compare it to a different mode in a different key that matches up closely to what I have, which is typically called "modulation".
This should give you a good jumping off point.