If you have no idea how to take apart the inside workings of a toilet, then search on You Tube. There are several videos of how to take out and replace the parts inside the tank.
There is a ballcock on a lever (big round ball ). The lever comes from a mechanism , and just above where the lever meets with the mechanism is a little sticky out white plastic thing . At the end of that there is a screw . That adjusts the flow . Pull chain with screw undone a little , then tighten it when you have enough water in the cistern. It shuts off water inlet by blocking the hole where the water comes in . Make sure that the hole is not blocked in anyway by something not meant to be there before making any adjustments. These things have a way of undoing themselves over time so its as well to learn how to do this yourself now. Good luck .
When I looked into cistern the water was spraying out at the top of the mechanism- a fine spray. I'm clueless what's wrong as it's been fine until this morning!
Any help is appreciated ????