I suggest that you make a short list of some of your favourite guitarists - those who are most influential in what you listen to, or why you decided to play the guitar, or those whom you admire.
After that do a quick wiki-search on them and peruse the information to see which one would be the easiest and most interesting to write about. Essays that are forced, concerning topics that do not interest you are the hardest and usually end up the worst.
If I were the one writing the essay - based on the information given I would choose the guitarist who was the most influential in the music scene, perhaps led an interesting life, and also who may have had a signature sound in the way he played - because those are all easy topics to work into an essay.
Once you decide on a guitarist - check out other sources for information.
But as the second person in this thread mentioned - it's really up to your choice, because it has to be someone who you find interesting or WANT to write about or writing the essay will be painfully boring and tedious.
^_^ Good luck.