If you're asking if you can lose 100 pounds in one year, then the answer is an absolute yes. Start today by searching yourself to determine why you gained this much weight so quickly. Are you depressed? Bored? Upset? I strongly urge you to understand this before you go forward. Also check with your doctor to ensure that you don't have a medical condition that is contributing to your weight gain.
Definitely start exercising. Start with 30 minutes of walking every day if you can. It's okay to break it up into a few sessions a day. Do whatever it takes. And increase to up to an hour gradually.
Whatever happens, go to the wedding with your head held high. I'm sure your family loves you whether you're 100 pounds or 200 pounds. You are worth more than a number on a scale.
Note: Remember that you should always ask your doctor before beginning any exercise programs, and this is particularly important if you've been sedentary for a while or have gained lots of weight -- especially if you're over 40 or have particular health concerns. If your doctor recommends a stress test, schedule it before starting your program.