Play a major scale. Start making riffs out of it. For the bridge, just choose the minor scale which contains the same number of #/b s as the major scale. That's the most basic way to write a song.
Or if you are after a punk sound (another easy writing genre), pick 4/5 chords, all of which are your basics (C,A,E,B,G, etc. - no #s, bs, min, maj, 7s, etc.). Then just play them in in descending order.
Grunge isn't too hard either. For grunge make about a 50/50 ratio between natural chords and flats - i.e. C,A,Bb,Eb in a vareity of orders will make a variety of styles of grunge.
For folk, try slight variations of fingerings. i.e D,Dm,F,Am will make a nice folk riff. For a folk chorus, add in a higher note like C, a mid note like G, and then play with the verse chords.
That's about the only styles I've tried writing. If you are looking for other tips, well, just ask other people. It all depends on the style!
Life is what happens while you're busy making other plans - John Lennon