how to potty train my 20 month old little girl she keeps taking her diaper off
How much can she talk/understand? If you can ask her why she takes it off, maybe you'll be able to work out if she's really ready.
If she takes it off just to get attention, maybe she's not ready.
If she takes it off because she hates the wet or dirty feeling, maybe she is.
Then look back at some of the other potty training questions on this site - Filletofspam and I gave some advice on one about a month ago, and I thougt we covered most of the ground then...
Best of luck!
http://www.funadvice.com/q/howtopottytrainyour_kid
That's the link to the question I mentioned, but if you type 'potty' into the 'search' box at the top of the page you'll find loads of related questions!
Everytime she goes potty and washes her hands, yuu should give her something that she wants (not sweets) and then she will grow up believing that going to the toliet by herself is a good thing