How do you set a good bass line for your songs? What do you base it on?

Usually you base your baseline on the root note of your track's scale if you want to keep things simple. If your write your melody first then you can play around the bass notes to compliment the melody. You can achieve that either by keeping the bass notes the same as the melody (in a lower octave of course) or writing a complimenting bass melody that supports or adds harmonics to your melody. Hope this was helpful.