Taking The Mic
To an extent any microphone will do - I've even made do with plugging headphones into a mic socket and talking loudly – it does work but obviously isn't ideal. Basically, the better the microphone the better your recording will be.
The best you can get for general use including speech is probably some sort of uni-directional (aka cardioid/supercardoid), condenser (phantom powered) microphone made by, AKG, Audio Technica, Beyerdynamic or Sennhieser (amongst others). The uni-directional bit is fairly self-explanatory even though it's not entirely accurate. Cardioid mics have a pick-up pattern sort of like a heart (yep, it's yet another ancient Greek derivative).
See diagram... (to follow)
For this microphone and to keep the audio quality of you should use an independently suspended stand/basket arrangement.
See pic... (to follow as well)
If I've found the right pic then you'll see a pop guard in place too (minimises heavy breathing noise amongst other things) although many better mics have a sort of built-in (you can't see it) pop deflector.
The phantom powered bit actually means the microphone needs a seperate battery source of some type away from the mic.
Of course even I don't take my own advice - my favourite microphoe for most things is my old Sennheiser 416 with an ABF3 power source - sometimes I have even been known to run this through a mixer before recording (listening through my trusty DT100s) - but most of the time I make do with my, on board battery powered, ME80 - a nice short rifle style mic cabled directly into a mini-disc recorder. I still use those cans though but they are quite bulky. I also use a properly suspended hand grip.
See pix... (if you're lucky)
The quality's not quite so hot with the ME80 compared to the 416 but very, very few folk will notice.
Back in the real world, try whatever mics you can find lying around though. Most will probably suffice especially if the audio's going to be compressed for web use eg. Guided tours etc. See http://prestongrange.org/audio.html. Try them out and if you're happy with the audio you get then that's just fine.
If I'm not making sense or you wanna know much more about microphones then a good starting point is probably http://en.wikipedia.org/wiki/Microphone
Anything Else?
If you want to go nuts then you could always throw in a remote mixer (such as the SQN I sometimes use) for level adjustment etc before the mic's signal reaches your recording medium of choice (more on this in a bit).