How to activate the self timer in my Nikon D90?

I just bought it and I'm still learning. I can't seem to find the option where I can activate the self timer. I found the settings, though. But everytime I press OK, it doesn't get activated.

Please don't rush into learning a camera in one day--this means you really need to read the manual. The option is there in the menu, but you have to find it. And looking for it randomly isn't going to help because you will have other questions in the near future and learning it on your own (it's an open book test, by all means), the answers will stick.

The manual knows the answer.

What you've found are the detail settings that change the way how the self timer works, but that doesn't set the camera to self timer shutter release mode.

Next to the top lcd there's a button with a timer symbol (the one above the AF button). Keep it pressed and use the thumb dial to change the shutter mode.

Note that there are two different timer settings. One of them also involves the remote symbol. That's the timer that can be started with the remote, but not with the normal shutter release button. In order to start it with the normal shutter release button you have to select the other one.

Manual pages 64, 66, 67.
Also see page 179, function c3.

Once set, press the shutter release and run in front of the camera.

When all else fails try reading the Owners Manual because 99% of the time you would be able to find the answer to your question sitting right inside the Owners Manual.