Is your toilet slow to fill ,noisy filling ,overflowing ,if yes here are some handy tips to change the ballcock.
First the water needs to be turned of, by law there should be a valve sited near to the toilet. However there may not be and the whole house or the water tank in the loft may need to be turned of.

It could be a traditional valve which works by turning the valve as far clockwise as it will go (approx. seven turns from fully on to fully of) or it could be the new iso valves which are turned on and of with a flat head screwdriver. Sometimes the iso valve or traditional valve when they are old develop a leak after turning them of and them back on which could lead to them needing changed..

A iso valve is turned ONLY one quarter turn from fully on to fully of a simple way to understand is if the valve is vertical then the screwdriver slot should be EXACTLY horizontal and if the valve is horizontal the slot should be EXACTLY vertical.

Now we need to check if the toilet is switched of remove the toilet lid it may just lift of but may of come of six inches or so and then a clip pushed out to remove a cable or if it is a push button type the button may need turned by hand anti clockwise or if there are two buttons one button may need pushed down and a screwdriver used to lift up the other button out of the way and then used to lift the other button out of the way then a screw under the buttons to be unscrewed (anti clockwise). Once you have the lid of place it very carefully on the floor.

If you are in any doubt very carefully put slight lifting force on the lid ,the insides to the toilet on push button toilets are very easily broken.

Now push down the float only a few drops should come out and then stop ,if the water is not switching of then the control valve may be faulty. All the water needs to be taken out of the cistern I use a hose to syphon the water out (suck on end of hose then stick in toilet) or use a sponge as any water left in cistern will spill on to the floor.

Now we need to undo the pipe from the ballcock I just use a adjustable spanner (clockwise to loosen ) and then undo the ballcock from the cistern (some modern cisterns require the whole cistern to be removed from the toilet to change the ballcock).

Now put in the new ballcock and tighten back up and fit pipe and tighten back up. Most of the time The new ballcock length should be the same as the old ballcock but not always if this was the case the pipework would now be to short or to long and would have to be corrected. The washer on the pipe should be checked (usually a red fibre washer)if it does not look in good condition replace with a new one, any plumbing or diy shop will have them. Or remove the old washer and add a good amount of white tape(other wise known as ptfe tape or plumbers tape).Care must be taken to make sure both the ballcock nut and pipe nuts are put on to the correct threads as being plastic it is very, very easy to cross thread .If the ballcock is cross threaded do not attempt to cut of the damaged part it will not seal, a new ballcock is the only way to fix it.

Now you need to check the float in the ballcock can freely move up and down on its on accord. Now the water can be turned on very slowly and a check for leaks carried out. If you are leak free check where the ballcock water height stops there is always a line inside the cistern where the water level should be. Now replace the lid.