Using a Nikon D200 with a new 18-200mm lens. When

Have a close look at the iris ring to see if any think is broking off
or try moving the lens when it is locked on the camera to see if you lose the fee if so it may need a new mount

Change your metering mode to spot metering. Focus on your subject with half-press of shutter button...hold that press while you re-compose the picture and press the rest of the way. See if you get better results.

Check your Custom Setting Menu, a1-a10:AUTOFOCUS.

Press Menu, select the Custom Menu Setting (looks like a pencil).
Select "AUTOFOCUS" and move to right.

a1 AF-C Mode Priority. Set to "Focus"

a2 AF-S Mode Priority. Default is "Focus", change this to "Release". This will enable the camera to fire anytime you press the shutter, regardless of if it's in focus or not. Use this setting if your having problems locking up, especially using some different lenses.

Try these settings. They may work for you.

Your lens might need to be recalibrated. Try sending it to PhotoTech Repair Service in NYC. They are Nikon authorized, and can do the repair under warranty if you still have it. Also if you join they're facebook page, they will give you a 10% discount.
Here is the facebook page link, it has everything you need to send it in.
http://www.facebook.com/pages/New-York-NY/PhotoTech-Repair-Service-Inc/102527215194

If you move from mode to mode, the camera will remember the settings from the last time you were in that mode and reset to them. This is handy if you are in shutter priority shooting sports at a high shutter speed, and then want to take a picture of something that's not moving fast, like the crowd. You just pop it into aperature priority with a remembered settings of a higher f stop.

I use this to shoot the scoreboard, which has a fairly slow refresh rate and usually comes up blank if I shoot it at a shutter speed higher than about 1/100.

First, if you can try the lens on another camera body, please do. See if the same thing happens on a different camera body. If it does, it's a problem with the lens. If not, you need to look at the camera. You can verify it's the camera by placing a different lens onto the body and see if you're experiencing the same problem. If it is indeed the body, I would suggest you take a pencil eraser and gently but firmly clean the contacts located on the lens mount. It's my experience (I also shoot with a D200) that often times when a lens isn't autofocusing, it's due to direty contacts either on the lens itself or the camera body. Cleaning them is a good start though and will narrow your issues down to the lens or the camera body. After cleaning, try the lens again to see if it's working correctly. If not, send it to Nikon in New York for repairs.

You could use flash. Depending on distance, you might want to go with one of Nikon's add on flash units. SB-600 is very cost effective.

You could also buy the excellent f1.8 50mm lens that Nikon makes. It's about $120 new. It's a prime lens (no zoom/telephoto) but it is one of the best lenses Nikon makes. It works very well in low light. There are also some other large aperature lenses, but cost goes up as f goes down!

Are you using the automatic mode or manual mode? Try checking the dial and see which mode you are using.
If you are shooting on the mode with the crossed-out flash symbol, flash will not be fired, and if there is not enough light, the outcome will be very dark

clean the lense.then lick my arse

I would recommend the 17-35mm f2.8D ED-IF rather than the G series