how do I keep power on after closing the lid

I certainly understand what you're trying to do as I do it with PCs all the time. I have a PC notebook at home that I leave running 24/7 for remote access. It's in a tight spot and really is just in the way with the lid up. Windows easily lets you select how the machine behaves with the lid closed.

Having said that, knowing how Apple designs their systems, I tend not to override their hardware settings. NoSleep is a popular option, but be sure to read the comments. I don't really see any issues per se... just that it may not run on all systems. Keep in mind that your system will stay running when you do this (which is the reason you're doing it of course)... so just be sure to open it up and shut it down or turn off NoSleep before you put it in a backpack, etc. so it doesn't overheat on you.

http://www.macupdate.com/app/mac/37991/nosleep

For what purpose do you want to do this? That is the real question.

If you leave the lid open and set the screen to go dark in 1 to 2 minutes and also set it to never go to sleep when connected to the AC power it is basically the same as having it awake with the lid closed, except the lid remains open.

The only way to have the system stay awake with the lid closed is to have an external monitor, Keyboard, mouse and AC adapter connected to it and then it will go into Clamshell mode when the lid is closed.

Joe Warning wrote:

I have 2012 Macbook Air 13".

I would like to close the lid without sending the computer to sleep.

I have tried to fiddle with "System Preferences" but I had no luck.

Someone help please.

Right so buy a portable music device instead of using your computer as a large iPod.

As stated in my first reply. All you have to do is set the screen to go dark at 1 minute and then walk away. In one minute the screen will go completely off. No need to fool with the brightness setting at all.

You may have to look for a third party app.

How to use your computer in closed clamshell - Apple - Support

Cause Apple Knows Best. Unlike the Windows side where MS Use To give the user choices. But that is changing also. Seems everything is going the Phone/Pad style where the user has only a few options.