Being one of those boys who went to the local borstal^H^H high school and then went on and did a mechanical engineering course, I've done this a few times myself.
As Nelly says, there are 3 types of tap, but you should be able to get away with just buying one - the taper tap - if all you need to do is thread a couple of holes in a bit of metal.
You can even get away without using a proper tap wrench - I just use a pair of quick-grips, but then I've done it plenty of times! The tap wrench will make sure that you get the tap level and apply even pressure so you don't tap at an offset angle, but with a tapered tap you shouldn't have this problem anyway.
When tapping, remember that there are three size holes - the size of the hole you need to drill first before cutting the tap, then the hole that is cut with the thread in it, then finally the size hole you need to drill out the thread.
If you look at this page, it gives you the sizes you need.
http://www.usq.edu.au/users/grantd/m...us/TAP_DIE.HTM For example, you need to drill a 5.1mm hole for an M6 thread, but if you want to drill out an M6 thread you need to drill it out to 6.4mm. However, not many people have a 5.1mm drill bit, so a 5mm should be fine.
Depending on the metal, it's always a good idea to cut a bit, then back it out. Keep doing this to free out the swarf you create otherwise this can get jammed and ruin the thread. Stuff like aluminium you can get away with just banging it straight through in one go!
You can pick up some very cheap tap and die sets, but you may be better off just buying the size tap you need. I've got a couple of Draper taps in common sizes and these are fine. Taps are a bit like drill bits - cheap ones go blunt quickly if you use and abuse them.
Take your time, use some scrap to test and you won't go far wrong.