How to fix my nikon D40?

Where do I start? The shutter will only fire in maual mode. If I put it in auto focus mode, It will not fire at all. I have cleaned the contact, and it still doesn't work. It's not the lens because I have put the lens on my other camera and it works. It is a kit lens. I want to fix it just because. I don't want to send it to nikon for repairs. Where do I start?

Try gently cleaning the lens contacts on the camera body. You can also mount the lens and while the lens is locked in place, rotate the lens mount back and forth a few times (the lens mount will rotate a fraction of a degree). This action may be enough to clean the oxide layer off the contacts. The D40 with kit lens has lens contact issues, even on a new D40.

Have you tried shooting during the day in full sunlight? The D40 will not take a shot if it can't acquire focus, which can happen if it is too dark for the AF system (e.g. Indoors), or if you are aiming at a smooth surface.

Have you also tried shooting in Program (P) mode? It is a bit like full auto mode, but gives you more control over certain settings like exposure compensation.

Start outdoors with the sun up in the sky. Set camera to Auto mode with AF on. Make sure exposure compensation (lightmeter) is at zero. Focus on something brightly colored 10 feet away and shoot it. If the camera fails, try without a memory card or better yet, a known working memory card. Format the other card first before shooting with it. If the camera doesn't work with a new card, there's something terribly wrong with it. You have two choices - send it in for repairs or sell it as junk.