First, try tightening the fittings with wrenches. Use one wrench to hold the brass fitting still (very important.) Use the other wrench to tighten the compression fitting (at the leak.)
If that doesn't work ....
Turn off the water at the the last valve before the leak. (Turn the valve all the way clockwise.)
Now - without seeing it, I can only guess - but here goes: Take apart the fittings where the leak is. (use two wrenches - one to hold, one to loosen.) Some water may spray out when you do this - as long as you turned the water off, a little spray is normal - it should stop.
Since I can't see it, it's hard to tell you what to replace - but there should be a little brass ring or ferule on the tube you removed. That's the part to replace (probably.) Get a new one and put it back together.
But my hunch is that you shouldn't have to replace anything - just tighten everything snugly - don't overtighten.