Probably the easiest way to cut plasterboard is with a sharp craft knife (Stanley knife). Simply score one side of the plasterboard

 then bend the piece to be cut the opposite way firmly

 and then cut the paper with the Stanley knife on that side and the two pieces will separate easily.

The above method is great for long straight cuts but if you need a cut out like the sheet below you will have to use a old saw. Cutting plasterboard will soon make the blade blunt so don't go using your best saw to cut it.

Professional dry liners are experts at cutting plasterboard and make it look extremely easy, cutting out a hole for a socket or similar is simply a case of scoring it with a Stanley knife and then knocking the required piece out with a hammer, but this requires practice!

Alternatively a rotozip or similar will make light work of cutting such holes-