When I started this journey it was because my cousin showed up on my doorstep having lost 90 pounds. I was ready then and there to sign up with WW because I thought that was what she had done. She laughed at me and said "all" she did was calorie count.
So that's what I did.
Everyone has to find what will work for them personally. For some people, WW is amazing. For some it's South Beach, others it's Atkins, and still others calorie counting. Some people choose to go simply whole foods and watching what they eat. Whatever you choose the most important thing is that it is sustainable...something you can stick with long term.
100 pounds is a lot to focus on right from the start, isn't it? I suggest breaking that down into smaller goals, maybe focusing on just five pounds at a time. Celebrate your small victories.
You've taken a great first step by joining here.
This place is amazing and you will find lots of valuable information by people who have found great success with various methods. But you will find that the one thing all the big losers have in common is sticking with it.