Brainstorming is done in two steps. First, make your list. Then go back and try to find patterns in the list and ways to group the ideas into categories. Set aside any items that do not fit into groups. The groups with the most items are likely to reflect the ideas that you can write about most successfully.
Mapping is much like brainstorming, but it is more visual.
When you map, start with your subject circled in the middle of a sheet of unlined paper. Next draw a line radiating out from the center and name a major division of your subject. Circle it, and from that circle move out to further subdivisions. Keep associating to further ideas and to details related to them. When you finish with one major division of your subject, go back to the center and start again with another major division.