In Romeo and Juliet, what does Romeo mean when he says, after killing Tybalt,"I am a fortune's fool"?
Your question goes to the very heart of why this play is such a tragedy as it touches on one of the major themes in the play - the extent to which destiny rules our lives and we are just helpless playthings in the hands of fate. Consider the situation that Romeo has found himself in: Tybalt has killed his best friend, Mercutio, whom Romeo is now honour-bound to kill in turn. However, he knows the decree of the Prince that if more blood is shed between the Capulets and the Montagues, the person shedding that blood is to either be condemned to death or must be exiled. Romeo, having married Juliet, recognises that forces outside of his control are once again forcing a separation between him and his new wife, Juliet. This is why these two lovers are described as "star-crossed", and this is also why, Romeo, when he hears news of Juliet's supposed "death" before he receives the letter of the Friar, shouts out "I defy you stars!"
This play thus recognises the force of destiny in shaping our lives and how futile it is to try and go against it - Romeo and Juliet in attempting to "defy" the "stars", sign their own death warrant, as it appears that this play shows that there are some romances and true loves that just aren't meant to be.
In the Oxford(2000) edition we find: "In context "fool" allows two or more still current definitions: (a) dupe or sport, (b) jester(therefore servant)." At Wikipedia we find interesting notes regarding fortune. You might try to find ASIMOV'S GUIDE TO SHAKESPEARE at your local library.