There are many different brands on the market, but we usually use tap magic. Some cutting fluids are made specifically for certain metals, so make sure you are using the proper fluid for the type of metal you are drilling. If you can't get a good indentation in the material with a center punch, then the metal is probably too hard to drill and you will only screw something up if you keep trying. It usually cost much less to pay someone with the proper equipment to remove a broken bolt than it does to pay them to remove a broken drill bit or broken tap and repair the extra damage caused by a drill bit walking off center or an unlubricated tap breaking off inside of a hole that has been drilled to the wrong size, then had a drill bit run in beside the tap and broken off in a half-assed attempt to remove it. Threading holes requires 3 things to do it right.
Taps can wear out and get dull edged.
Again, proper cutting fluid. Check the chart below to find the right size drill bit to use with the tap size you are working with. Taps come in sets of 3. Start the threads about a turn and a half, add some cutting fluid, then run the tap in a couple more turns. Its is a lot easier to run a tap straight with a tap wrench than a crescent wrench.