It has the right key in it turns very little but will not let the cap off.I put wd40 in keyway but still not opening. Anyone have any ideals to get cap off with out destroying it. Thanks

spray  rust inhibitor into the lock and keep trying it  be patient until the oil can do its thing.  
doug

Any recommendations on rust inhibitor spray?

I had a problem with the fuel locking cap on my 1975 DT400 a few weeks back.
If I were you, I would drain the fuel, take the tank off the bike, put the tank upside down, supported on makeshift supports and spray WD40 up into the lock, using the thin white tube the cans come with. Spray for a couple of seconds, give the key a wiggle. Repeat that until all the crap comes out of the lock. I went through half a can of the stuff until there were no more bits coming out. It makes a bit of a mess. The bits weren’t just dirt, they seemed to be very small bits of black plastic. I guessed it was from either the remains of a mechanism to stop you taking the key out of the lock when it is unlocked, or a flap over the top of the key way to stop the rain getting in (although there is a big metal flap for that).
If that does the trick, put a small amount of light oil in the mechanism to make sure the key turn smoothly.
The main problem that I had with mine was that I had taken the cap off the bike and re-built it incorrectly. When I put it back on the bike, it locked on by itself and I couldn’t twist it back off. I tried everything to get it off, then, I’m not too sure why, but tried pulling it up (towards me), then twisting it and it came off. I was just about to give up hope and get the big drill out!!!
If the key is worn, or not completely flat (ie, it may have been bent, the flattened, like one of my ignition keys) it may be worth ordering a new key (using the lock number) and giving the new key a try?
Do all you can not to break that lock!
 

I use PB Blaster, not so much rust as it is an electrolysis effect between the brass tumblers and the "pot metal" that the cap base is cast from. Spray, let set over night, spray some more. And firmly, yet gently trying to turn the key in the lock. Additional spraying flushes it out.Always worked for me.

Just to clarify, the lock turns about 1/4 turn.  Cap twists off.  It's not like a lot of bikes where turning the key makes the cap pop open. You can leave unlocked.  In Jammyyammy's cap thread I posted the alignment of the key slot to the rivet when unlocked.  Long day and can't remember right now.  Sometimes cap gasket is just stuck if it's been sitting and cap just needs a twist.  Make sure it's unlocked first.

 

Cap turns very little and key turns very little. The cap turns maybe eighth inch or less and the key eight inch if not less. But will not come off. How much is key suppose to turn?

RUSTED SHUT

Take a extra large pair of Channel Lock Pliers and a heavy towel ( rag ) and twist the cap off in the correct direction. I had a 1975 - 1976 DT 250 fuel tank cap I bought removed in minutes. Yes to WD - 40. If the top part is rusted the bottom is rusted too. The tank I bought holed in several spots from being soaked in Vinegar several days.

a

"How much is key suppose to turn?"

As Mark stated in the post above yours, about 1/4 of a turn - say about 90 degrees.

If key slot is lined up with rivet, it's unlocked.  Only turns 1/4 turn.

Unlocked