There are various inlet valve types around now as ball cocks were done away with. Realistically you are likely to have one of two main types. Twyford usually used an inlet where a small cup is pushed up by the rising water level. This cup is on the end of a length of plastic which is attached to a plastic arm.
To adjust: you should be able to turn the length of plastic attached to cup. You need it to be further into the cistern. Usually these need 1/4 turn which allows you to move the cup further down then 1/4 turn back to lock it.
If it is not this type then there will be a float around the inlet which just has a plastic screw to turn clockwise to adjust-simples.
Hope this helps.
