This is where a small amount of music theory can set loose your creativity. In about 30 minutes of googling "chord progression theory" or "diatonic chord progressions" or "circle of fifths chord progression" will generate all kinds of ideas for you. And the great thing is that Band-in-a-Box will give your ears immediate feedback on whether your idea works.

Another good starting place is to use some of the demo songs included with Band in a Box as a starting point. Erase the melody, transpose to a different key and start writing melodies. You can't copyright a chord progression, so you can even take your favorite group's songs, erase the melody and use their progression as a starting point to write your song. It is a great way to learn what works in almost any genre of music.

Good luck.