As a quick guide, most electrical items have a sticker or information panel on the back ( PC, washing machine TVs, cookers) saying how much power they use.
Most PCs have a 200 Watt power supply but generally use much less. All power ratings given are as if the unit is being used for 1 hour. Your electricity meter also shows how many Kw per hour you are using. If your meter is one of the older ones with a visible spinning disc in the window, note how fast it is going then get someone to switch on the kettle or cooker an watch it go!
So in the case of your PC taking an example amount of 200 Watts per hour it would take 5 hours to use one unit (1Kw hour) of electricity.
Your electric bill will show how much your supplier charges for each unit of electricity.
As a rough guide items which heat things up use the most electric. So cookers, immersion heaters, electric fires, fan heaters,kettles, electric showers etc all use lots of electric. If you need advice how to save energy try the energy saving trust.
http://www.energysavingtrust.org.uk/