The major pentatonic scale is an excellent scale to learn. Not only is it simple, but it is also very useful for bass lines and solos in major keys. It should be one of the first bass scales you tackle.
What is a Major Pentatonic Scale?
Unlike a traditional major or minor scale, a major pentatonic scale has five notes, rather than seven. Basically, it is a major scale with some of the trickier notes omitted, making it more difficult to play something that sounds wrong. Plus, it makes the scale easier to learn.
This article goes over the pattern of a major pentatonic scale in different hand positions on the fretboard. If you haven't read about bass scales and hand positions, you should do so first.