I have a small garden which is about 20 feet long and 15 feet wide.
It's full sun nearly all day

Well, that gives you a lot of options. Flowers, culinary herbs, and
vegetables mostly like full sun (with a few exceptions). You'll want
to put several paths through it so you aren't walking on the beds.
But you can figure out whether you want rectangular beds (about 36
inches wide) or meandering paths, or what.

Assuming you have some time to spend in it, you don't need to worry
quite as much about making everything low-maintenance as you would in
a bigger one. Although 20 feet by 15 feet is big enough that you'll
want to avoid just ignoring it and letting all the weeds grow up
(putting in enough plants to shade out the weeds can help; mulching
can help; etc).

I don't know, there are a lot of books about how to lay out a
garden... Did you have any particular questions in mind?