WaltzingMommy is correct. Get in to a studio ASAP. Tons of times I have couples come in to learn a wedding dance two weeks before they get married. Needless to say they find it eye opening.
Here's my suggestions:
Tell the studio what you want to do, be it dance together or with your parents ect.
Take the music you want! And have another song picked as a backup. Some people find the music they like is a Waltz, but one of them despises the waltz, so a good backup is needed.
You need to both practice in the shoes you will wear on wedding day, especially the bride in heels. Trust me, this helps.
Know what type of dress you will wear. It will afffect what you can/can't do in the dance depending on how it's made.
Consider your dance options. An almalgamation will give you a set routine. You can go as far as a whole routine for the song, or just an nice entrance and then freestyle the rest of the song. Opinions vary as to what is better, choreography or freestyle. Talk with your instructor.
Knowing the size of the area you'll be dancing in is a good thing when approaching a studio.
Lessons are not cheap. Consider a smaller package strictly covering the dance you end up doing. Check with friend's relatives and have them buy the package as a wedding gift!
I personally had a couple buy a full foundation level package, actually a gift from the parents. They used 7 hours to learn Foxtrot, in which I agreed to give them an entrance and a couple advanced moves. They then returned after the honeymoon to finish the package out.
Grats and have fun!