If you are asking about the end of the tubing you purchased: I don't believe the threads on the tube nut are NPT. They are straight SAE 1/2-20. You are not sealing a pipe thread seal. You are clamping a flare into it's seat.
Furthermore, you don't need a barbed fitting. Just slide the nut back, slide the hose on and clamp. The flare is your barb.
If you really want a fitting, take the line to the store and try fittings to find the correct thread.
If you are asking about what to do at the tank outlet, need more info. Tank probably has 3/8 NPT female bung.
