First you need to determine exactly what is leaking, one of the joints or the valve itself.
If it's the valve, follow Michaels advice.
If it's the joint between the fitting or tube and the tank, you problem isn't as easy to repair. Pipe threads are tapered so that when you make a threaded joint the threads make the seal. If your fitting threads are bottoming out in the tank, call the vendor you bought the tank from and ask what to do, it sounds like the threads in the tank are not tapered.
I'm not a fan of using teflon tape on gasoline line joints. First, it doesn't seal real well, second a piece of the tape can come loose inside the fuel line. I now use the Permatex Hylomar sealant, developed by Rolls Royce for use in carbs. There are other gasoline resistant sealers out there, too.
