metallica and aerosmith = something new and different :D
Ok, pop rock is pretty easy to write. Most pop rock songs are written in the Mixolydian (major sounding) mode or Dorian (minor sounding) mode. Use E Phrygian if you want to be metallica. Hopefully you know how to play these scales, and how chords are constructed in these scales. From there it's pretty easy to figure out a harmony. A three chord harmony in D mixolydian could be Dmaj-Gmaj-Cmaj.
A tool for getting inspired to write catchy lyrics is a book or list of proverbs. This is good cause alot of popular songs come from popular sayings and quotes. A another good thing to do is listen to the social world, and when you hear something that rings back to you write it down.
Once you got a few ideas, try singing them and playing chord thats sound good with how your singing whatever line you have. Catchy is usually something that is familar or very easy to remember, and often comes from having music that goes with lyrical ideas that are catchy. Also catchy stuff in pop music is usually something that is fun to play, so have fun too.
To practice writting forms and a complete song, the best way to learn is to learn from the bands you like. If possible, download the midi of a song you like. Play it on a midi sequencer, mute out the guitar tracks and vocals and leave nothing but the drum track. Then try writting your own tune over the drum track. Make sure your not writting the same song or anything similar, do something different. Write a complete song that way, and you'll learn alot. If your having problems thinking up something and it doesn't influence your writting to copy the original, leave the bass track in to. If you do so, after writting a couple of tunes, take out the bass track. Then once you get a feel for how songs are constructed. Start writting your own drums part or download loops of the net, and you'll be on your way.
BTW, in the styles your inspiring for, the minor scale is a good choice. If it sounds too gloomy, play the minor chords that sound too down as minor 7th chords. Or transpose the song from the minor key to the major. Ex. A minor to A major.
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.