Bonita:
+1 what Ken said.. You are smart, learning to use one program at a time, then building on it later. You will do fine! Soon, you will be able to explain the manual to us all.
Hmmmm...50mm lens; I asssume it is the famous Nikon f/1.8? Good one. But, remember, it will give you a 75 mm equivalent on your D 90, essentially a short tele, which you may find a bit restricting indoors.
A wide angle lens or the 18-55 "kit" lens may be just what the doctor ordered. It is a marvelous little optic!

Don't worry too much about the f/stop not being as fast as the 1.8...that's what is marvelous about digital: you can jack up the ISO to 400 or even 800 with excellent quality. Set your camera for the highest quality, and large images. Choose an aperture that will give you some depth of field, so the focus doesn't get easily farbled.
Bounce flash is a goodness, particularly for this sort of activity.
Now: If the photo gremlins strike, and you are not happy with your results,

AUTO, P or one of the "canned programs", as depicted by the small icons on the knob that lives on the left side of your D90 have pulled many "pro" fannies out of the fire. They will not admit that's what they did, tho; no, they won't!
Let your camera sweat the hard stuff...that's why you paid all that money for it, right? AND there are times when it is perfectly O.K. to use it as a big, expen$ive point-and shoot! You point...it shoots!
Practice a little with your choice of settings before the big day..waste some pixels...and have the camera pre set for whatever you think the situation may be before you arrive...saves some anxiety.
The "PARTY" program is a good one.
Act like you know what you are doing...few will question you, with your impressive,"big" camera.
Shoot, then chin in hand, peer at your lcd screen thoughtfully; evaluate, adjust as necessary, repeat, and you will do fine...just don't forget to return the settings to "normal", whatever that may be for you when you are through. (Don't ask how I know this.)
The best advice is go and have fun! Shoot lots; The more we shoot, the better we get!

You are gonna do fine!