Because of their requirement for small sized engines, the (we call them trimmers here in the U.S.) 'strimmers' are powered by 2 stroke engines. It's not neccessary to go into the workings of the engines, but they do require mixing of oil and gas since they don't have any internal lubrication. The mix ratio varies with the manufacturer but usually a standard 3 ounce (around 20:1 to 80:1) can will be used for one gallon of regular automobile gasoline. (Here's a calculator:
http://www.csgnetwork.com/oilfuelcalc.html ).
Simply pour the container of oil into the container of gas and 'swirl' it around just a bit to mix. Be sure to consult the owner's manual or the place where you purchase the machine to determine the exact ratio. Too little and you'll damage the engine, too much and the neighbors complain about the blue smoke... Best of luck!