Riding on the back of a tandem is a very unnatural experience for a cyclist and even a non-cyclist. Anyone who plans on "captaining" a tandem NEEDS to take a spin on the second seat as stoker behind -- ideally -- an experienced captain before taking over the "helm" with whoever it is they intend to ride with as their regular stoker. This gives the prospective captain a great deal of appreciation for what their stoker will be experiencing and it WILL alter how you think about what you're doing on the front of the tandem. Moreover, it will also reaffirm how important communication is with regard to establishing your stoker's trust in your piloting abilties.
As others have suggested, if the person loaning you the tandem is an experienced captain then by all means see if he'll take your partner out for a ride, during which time he can instruct her on what to expect and how to coach you when you take over the helm. Also, if he's willing, ask if he'll take a spin on the tandem with you as captain so that he can critique your skills. Keep in mind, riding a tandem with a larger stoker (height more than anything) is much more challenging vs ones of smaller and shorter stature.
Back to you and yours.... I can't over emphasize that the keys to success for tandem teams are building trust and learning to work as a team. I've always maintained that riding a tandem is very much like ballroom dancing. Someone has to lead and someone has to follow but both need to compromise a bit to dance well together. "Negotiating" those compromises requires a lot of verbal communication with both riders taking turns being in transmit and receive mode.
Also, never forget that when you captain a tandem you are asking your partner to give up all control and to place their trust in you for your collective safety and comfort. Therefore, you'll want to tell them what you are doing before you are doing it throughout the first couple of rides, i.e., shifting, bump, left turn, stopping, etc... At the same time, you need to be attentive to what they are saying, what they aren't saying (dead silence is not a good thing) or what you're feeling from them through the bike. All the while you'll also have to work through compromises on pedalling cadence and riding style, particularly if you are both active cyclists. In fact, in some cases captains who take on non-cyclist stokers have an advantage in that the stoker's haven't developed a riding style or tempo.
Well, I could ramble on but you get the gist of my comments. As for some of the other fine points here are links to some additional articles that you might want to read before tomorrow...
http://www.gtgtandems.com/tech/newriders.html
http://www.sheldonbrown.com/tandem.html
http://www.gtgtandems.com/tech/propmethod.html
Some of the content is redundant, but if nothing else it should reinforce the notion that certain methods have proven themselves to be better than others.
There is quite a bit more available on the Web with regard to tandems and most of it is linked off of my Web site:
http://www.TheTandemLink.com
The "first time tandem team" information begins at this link:
http://home.att.net/~thetandemlink/tandems.html
Last bit of advice.... have "fun" on your first ride(s). You learn more by taking it easy, discussing what you're both feeling and working through your differences than you can by going hard at it from the get-go.