i have an image that i really like, but i want it to be black and white. how do i change it?

You can't.. not on paint.

you need paint shop pro to do that... i`m pretty sure.

or photoshop too,

yeah i dont think you can do that in ms paint....but you can with ms word and paint. copy the picture and paste it into a word document. a little toolbar should pop up that looks like this:
Click on the second icon from the left...

and click on gray scale. Et voila! (if the picture toolbar doesnt pop up, go to View > Toolbars > Picture

To make it extremely simple for you

Play around with the other tools on the picture toolbar to adjust brightness and contrast. When you're happy with how it looks, take a screen shot (print screen sys rq at the top of your keyboard) and paste back into ms paint.

or use IrfanView