How do I fix my iMac if it won't start up?

What exactly do you mean by "won't start up"? When you press the power button do you get any sound, fan or lights anywhere? Have you verified that the outlet it's plugged into is hot?

How you fix it will depend largely on why it won’t “start up”…

Could be anything from a very simple fix to complete replacement of the machine…

You’ll need to provide just a tad more information.