First of all ensure you actually have a computer and haven't confused a microwave for your pc.
Not a computer:
Next ensure that it is plugged in at the mains. There is no need to lick the plug before plugging it in. Below is a standard plug for the UK and Malaysia.
If you are in the US or Canada then you will have a plug set-up like this below. Please don't try to plug a US plug into a UK mains. Though this shouldn't be a problem unless you have a very long wire.
Please ensure at all times that it is a plug that you are trying to insert into the mains.
Before you actually switch on your machine ensure that no pets are inside the computer. Cats are especially notorius for getting inside your machine trying to "fix" things.
Next press the 'ON' button. This is usually located at the front of the computer. This needs to be pressed firmly, saying "Make it so" as you press it is optional.
*Note: This is my guide so far, it's still a work in progress but I'm hoping to get it moved to the Guide Section. Any comments would be helpful, particularly from other countries because you have wierd plugs.
Special thanks to Rossco for his invaluable help with US/Canadian plugs and thanks to Maskawaih for help with Malaysian plugs.
Thanks also to my Health and Safety advisor, Arcadia.