A good place to start when your creating your own song is to take either a melody or a riff that's relativaly simple and has some kind of character to it. Usually a good riff or hook will stay in your mind for a good while. That's how you know its strong. I like living with the idea for a few weeks. If I notice that I keep playing that part, everytime I go to pick up my guitar and enjoy playing it, then at that point I start structuring a song. I look for a common way to keep the hook in the song - whether it be a verse or a chorus with lyrics and then at the point I like to let the idea try and develope itself as apposed to try to manufacture the song. I hope that helps and gets you started ?
Remember to have fun and if you have any more questions let me know ?
Aldo