Get drumloops or make your own with drum programs, a good one is fruityloops. Mess with a making drum parts. Here's the path I start with, it's called part-writing.

STart a new drum part.
I usually begin with the hihat on every beat. Then I mess with the tempo to see how fast I want the song to go.

Then I'll just sit there and listen to the hi-hat, getting the tempo in my head. After I get the tempo in my head, I start thinking up a beat with bass and snare. I'll lay that down.

Next I'll think up the rhythm I want to the song. Using either bass, guitar or both. Just think up a rhythm add that too the mix. Here you should get your chordal structure, if not I just figure it out.

Now you have the entire rhythm and chordal structure done, rest is cake.

Next part is open to what comes first. Usually a melody line that go with the rhythm. I usually start humming it until I get it completely stable in my mind then I figure it out on guitar. Now you got your melody, I usually base an entire song on that melody, but you may have your own way and may to change it throughout the song.

Now that I got my melody, I begin to build a riff that goes with it. Usually just hum it til it's in my head then, figure it out then lay it down.

After that, you just go with the flow. Start adding harmonies, changes, maybe a solo, whatever you want the song to be. Once you got the rhtyhm and melody, it seems like everything else just falls into place.

This is still just my way, you probably all have different ways but maybe this will help you think up your way. Sometime you may start with a cool guitar you came up with, but I think the process of part-writing works really well.