If you are going to give a presentation of 15 to 20 minutes duration, then it had better be on something you know well, or you will run out of things to say after about 3 minutes and look really foolish. Normally, a presentation of 20 minutes would require me to work for about 30 minutes per minute of actual presentation - and that is on a subject I know well. So, I would be looking at two whole working days prep for your little talk... If I didn't know the subject, make that two working weeks.
There are some tips on presenting here:
http://www.garrreynolds.com/Presentation/prep.html[
^] but do not expect it to be simple - you have to put a lot of time and effort into getting it right beforehand.
Since we don't have a clue what you know, we can't begin to suggest a subject.