If the leak is on the feed side into the tank, and the stopcock is between the pipe and the mains then yes, turning off the stop cock will isolate that join.
Once you have turned off the stop cock there will be no water pressure to cause water to spurt out. There will be a small amount of water in the pipe and this could leak out so put a towel or something below the pipe when you undo the nut.
Be cautious though because if you have not used the stop cock much it may be stiff, or not turn the water fully off.
I would think the most likely cause of a leak is the washer holding the fitting to the tank rather than the nut. If the washer is still intact then just tightening the joint may work, but it may be better to replace the washer.
Alternatively you could try a couple of warps of PTFE tape around the end of the joint where it goes into the fitting and around the thread.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!