Start by writing down one of your main ideas, in sentence form.
If your main idea is "reduces freeway congestion," you might say this:
Public transportation reduces freeway congestion.
Next, write down each of your supporting points for that main idea, but leave four or five lines in between each point.
In the space under each point, write down some elaboration for that point.
Elaboration can be further description or explanation or discussion.
Supporting Point
Commuters appreciate the cost savings of taking public transportation rather than driving.
Elaboration
Less driving time means less maintenance expense, such as oil changes.
Of course, less driving time means savings on gasoline as well.
In many cases, these savings amount to more than the cost of riding public transportation.
If you wish, include a summary sentence for each paragraph.
This is not generally needed, however, and such sentences have a tendency to sound stilted, so be cautious about using them.
Once you have fleshed out each of your body paragraphs, one for each main point, you are ready to continue.