How do I repair a single knob faucet, bathtub drip?

A sungle bath fauicet makes use of a cartridge that is behind the handle and esctcheon plate. Remove both of them and tell be what you see? Is the faucet Moen or what?

How do I repair a single knob faucet, bathtub drip?
I have a moen faucet. Behind the control plate is a plastic piece inside of a pipe. I can't take the plastic piece out. Are there springs or seals behind it? Thanks for your help

Does it looks like this

C:\Documents and Settings\00508\Desktop\8334d1210072918-no-hot-water-master-bath-moenparts2.pdf

Sorry

Along with the diagram Afaroo gave you here are the instructions on how to replace the cartridge. Installation Help / Animated Tutorial - Installing the Moen 1225 / 1200 Cartridge (http://www.moen.com/consumersupport/installation/moen0133/moen0133.cfm) But before you do that, turn the water off and pull the cartridge out. Now turn the water back on to flush out the supplies and valve body. Reassemble and test. If it still leaks then you can replace the cartridge. Good luck, Tom