Just come up with ideas, brainstorm and then see what comes together. One program which has helped my chops and songwriting IMMENSLY, is the Power Tab Editor. It's hard to use, but once you get the hang of it, it will work miracles for your playing. There's a huge community of all of those who use it at, www.powertabs.net
You can get the shotware download link there as well. It allows you to write out tabs, and plays them back to you. It's like an extra guitar player to play whatever you want!
Yeah, writing a song is something personal, just remember to write down your ideas, and try to start thinking more of composition, making something come together as a whole. :D