Most of the monologues you'll find on websites will be really terrible and written by a twelve year old who is "deep" or "apathetic." You want to either:
read a play, or several; or
find a website that has monologues taken from ACTUAL, PUBLISHED plays (try Monologue Archive)
But really, read a play. Pick one you know the name of. Get your teacher to suggest one. Please dont just pick one from the internet.
Actually, i was having the same problem until i looked up men's monologues on google. i'll post the website. hope it helps, this is my monologue which i'm doing for my IP
For girls....
Joanna Murray-Smith wrote 6 monologues which are hilarious for the actor Caroline O'Connor.
The book which the monologues are in is called "Bombshells"
You Tube it.
The monologues range from a teenaged girl in a talent show - to an old widow. They're all funny.
I wouldn't trust website monologues personally.