95 Ninja 250r gas leak?

A "peatcock" is actually a petcock. :) It is basically a shutoff valve that attaches to the bottom of your fuel tank. I've never owned a Ninja, so I'm unaware of the exact process that you're going to have to take to replace it, but if you get the part, I'm sure it's not a hard process, most bikes aren't and I've replaced a few on both Harleys and sportbikes. You're going to have to take the fuel tank off and drain the fuel out of it. The petcock should screw into the bottom of the tank, either directly, or using a jam nut to keep it in place. My best advice to you is do some research and look online for a visual reference. There are plenty of Ninja forums, or general sportbike forums, which usually have how-to sections written by guys who have done the repairs before.