What if I have other information on my external drive and time machine erases it?

Time Machine does not destroy data on source devices. If Time Machine runs out of space on the destination drive it will erase the oldest data first to make room for newer data. The source drives are not touched.

Time Machine can co-exist with other data, although it's better to put it on a separate partition if that's how you want to use it. Time Machine will not erase the other data, just TM backups when it gets full.