A guest speaker can make a normally dry conference seem much more interesting, and the speaker can also attract attendees who might otherwise not be interested in going. If you want to hire a guest speaker for your conference but have never arranged something like this before, it's not that difficult. However, you will have to arrange for more than just getting that person up to the podium.
Be sure your budget can handle the whole trip. Unless the speaker lives in town, he or she is not going to pay for plane tickets and lodging to go to your conference; those costs would eat up the pay the speaker is receiving. Budget for the honorarium -- the speaker's fee -- plus travel costs, lodging, and a per diem for food and incidentals. (A per diem is a daily amount issued to help the visitor buy meals and things like toothpaste to replace what might have been taken away at airport security, for example.) Many organizations also issue taxi vouchers for speakers who are not renting cars; if your organization won't do this, ensure there is a local travel budget as well.
Who makes the lodging reservations can vary, but it's either done by you -- choose a reputable place that's within your budget -- or the speaker once you note what the housing budget is. If you make the reservation, be sure you adhere to requirements like non-smoking; also look into corporate rates that could get you a discount. You'll need to sign up for these, and hotels and motel chains should have more information.
If the visitor wants to arrange for his or her own lodging, that can be done, but be very clear that there is a limit of X dollars per night, and anything over that (including tax, if the tax puts the amount over) is something the speaker would have to cover. The upside to that is that the speaker can choose a level of lodging that is to the speaker's tastes.
If there are any costs that the speaker has to pay up front, such as hotel costs because the speaker is making his or her own reservations, reimburse the speaker after the conference promptly. The speaker should come away from the conference with the fewest budget dents possible, and reimbursing within a week of receiving all the receipts is best.
It sounds like a lot at first, doesn't it? Take it one step at a time, and you'll find that getting a guest speaker is no trouble at all. Contact a speaker service, like eSpeakers, for more help.