I would suggest using a lavaliere (wireless) microphone - any name brand (i.e. from a professional audio equipment company not an electronics store) will work fine. A good mic costs a minimum of $100.00. Handheld is also fine but may be more a hinderance for your talks. You can browse www.sweetwater.com and pick your gear; they have helpful tech support also.

As far as recording setup/ software you should get a portable digital recorder (cost: few hundred dollars min) with an input that your mic can plug into. You will be recording to the portable device and exporting the audio files to your Mac/GarageBand.

When you buy this gear, you will of course need to speak with an audio professional to make sure all the items you are purchasing are compatible.

Good luck!