Are you comparing Brautigan's poems to one another, or his poetry to someone else's? Sounds like you mean the former. Either way, getting started can be difficult; it can be helpful to make a chart. To be able to see the main similarities and differences, try listing the main features in one column and then in the next column, make notes on how each poem incorporates or digresses from that feature. You might want to color code the similarities in one color and the differences in another.
Give that a try and see if it helps you get started!