How do I turn on my iMac without the Keyboard?

There's a power button on the back of most recent models of iMac.

As you look at the display, the button is usually on the back side, behind the lower left corner:

Various Apple keyboards can be connected directly via USB-C or USB-A, depending on which iMac and which keyboard and which ports are involved.

If you're using a charging cable with your keyboard and iMac, plug that into the Mac and try the keyboard.

If you don't have a matching connector, an Apple-branded dongle or cable might work as an adapter.

iMacs have a power button on the rear of the screen. You use that button to turn it on.