How to fix a leaking pipe?
Well those stupid infomercials would say to "wrap it in flex tape," don't do that, or wrap it in rags. (Fiber fix would be better then both, but please fix it properly.)

That looks like a copper compression fitting. As such just tightening it probably will not stop the leak. There is a small chance that will work, it may have just worked its way loose over the years so start by simply tightening it, a little. Do not over tighten it. After a compression fitting is tight over tightening it will only make it worse.

Unfortunately if it still leaks it is a bit of work. Turn off the water, open up the fitting and replace the brass compression ring in side there. The trouble comes in getting the old ring off. You will either need a compression ring puller or a compression ring cutter (a rotary cutoff tool can be used, but be very careful not to damage the pipe).

With the new ring on, tighten the fitting down it is about one turn after it makes contact.