How do you write a good first person monologue?
My suggestion is that you read as many of Shakespeare's monologues (also known as soliloquies) as possible ... and then begin to develop your own.
http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=hamlet+monologues
There are many links here that will take you to specific places in Shakespeare's plays.
If you have trouble understanding any of them, go here:
http://nfs.sparknotes.com/
Find the title, act, and scene of the soliloquy you want to read -- it'll be in Shakespeare's language on the left and in modern-day English on the right.