They're cheap and very simple to replace.
Depending on what type of connection you have between your cistern and the wc itself, you need to decide whether you need to turn the water off. If it has a plastic pipe going to the back of the toilet you shouldn't need to. Just tie up the ballcock in the cistern then flush the toilet. You can now unscrew the large plastic nut by hand (at the back of the WC) and then unscrew the screws which secure the wc to the floor.
If the cistern is close-coupled it's a little more awkward but still within the capabilities of a DIY virgin. There should be a little stop valve on the copper pipe leading to the cistern. This will look like a shiney silver bit of pipe about two inches long and has a nut at each end. In the middle there should be a slot for a screwdriver head. Turn this 90 degrees (so the slot goes across the valve body) and this should turn off the water supply. If there is no silver valve, there may be a stopcock (a tap) on the pipe near the cistern. Failing that you'll just have to turn off the water at the main stopcock (usually in the kitchen under the sink). Flush the toilet and then unscrew the screws holding the cistern to the wall and the WC pan to the floor.
Now it's simply a case of gently pulling the WC away from the waste pipe and replacing the connector. Put everything back as it was and Bob's your uncle.