Welcome to Audio 101!
So you want a lav mic. Great, but how are you going to get the sound that mic can pick up to the camera?
Well you have two choices really. Hard wired or Wireless. This transmission from mic capsule to record device is really where its at. And there are certain factors you must consider before even buying a mic.
You didn't tell us much other than the presenter will be standing up in front of some sort of white board giving a presentation. Are they locked down to that area and promise not to move much? If so then hardwiring the mic can work. Its the cheapest and possible the most secure way of transmitting what the mic is picking up back to your record device.
But there is a down side to being hard wired and that's the limited mobility of that hard line. If you have a speaker or presenter that likes to be unencumbered and like to range all over the place then a hard wired mic is really going to be a pain for everyone involved. The cord will need to be managed, if you are wanting a wide shot and don't want to see the cord then that's tough, all sorts of little hindrances are involved with hard wiring someone all the way back to the camera/record device.
The other solution I mentioned is a wireless system. The presenter wears a mic connected to a transmitter (Tx) and the receiver (Rx) is connected to the recorder or camera. This way the presenter is free to roam over a wider area with no dangling cords to get caught or tripped over. Or be seen. But the wireless transmission isn't always a solid connection. Wireless interference can be an issue in some places. High RF environments can be troublesome for sure. With enough equipment and skill it can be worked around, but that can also be expensive. So be aware of that. They are also more expensive than the wired option.
I have done audio recording for TV shows for the past 19 years and have seen a huge advancement of RF mics. They just keep getting better. I own a large selection of Lectrosonics wireless mic systems and can usually get solid link. Some places are tough though, but those are extreme situations.
As for a simple one person presenting at a white board, I would use a wireless system over a hard line most of the time. The simple fact of the presenter being tethered changes many things. It puts a physical limit of the presenter and that can affect some people's presentation. They start to over think things and it can become a problem. Where as the wireless system won't limit them and in many cases they will forget they even have a mic on at all. I've seen it many times.
So your budget is the first thing you need to get wrangled. Find out how much you've got to spend. That will truly be your best guild in this situation.
If you have more than just a couple hundred dollars then your choices start to open up.
I will suggest that you look into a Sennheiser G3 system. They can be had at B&H Photo, Amazon and on eBay for around $500 to $600 for a complete system (including a lav mic in the kit) that will last you a very long time. I still use my older G2 system on an almost daily basis. That's another option, look at good used systems. They can be found. One of my favorite places is www.trewaudio.com
They have locations in LA, Nashville and two stores in Canada. They have a consignment department and they list all the used gear they currently have for sale on the web site. I cruise through that stuff all the time looking for other bits and pieces that I might need. They have lots of great, tested, used gear. And they are completely reputable. So you know you'll get solid used gear. Maybe you can find a good used, older G2 system for a lot less than the new price. But that new price isn't a lot for a very solid bit of wireless tech.
Then, after all that, comes cables, mixers and all the other crap that production audio involves. HA! You were the one wanting to head down this Rabbit Hole! How far does it go? Hard to say... 
