Power meter + Power factor meter

I'm doing powermeter project, i don't know how to calculate the power from current and voltage sensing.. I have no idea to get the input and then send to PIC18F4520, then the power/power factor (according to user selection)

Also facing problem on pic programming, how to take a selection using 8 push-to-on button, that will execute the command and using if else.. the thing, will act as menu that display on LCD 16x2..

Menu will be

1) current
2) voltage
3) Power
4) Power factor

using PIC18F4520.. with mickroC software..

Guys, do have a proper reference or a kind of previous power factor meter project? if so, i hope u can give me a bit reference how u do that..really need your help...

ps: sorry for my first that already need your help..
pss : sorry for my bad english also..

thanks for your future reply..

regard
Mr.Am

I would advise you to look into the definition of power factor in wikipedia:
Power factor - Wikipedia, the free encyclopedia
There is an interesting section on measuring power factor.