Welcome to the board!
You are heading in the right direction by admitting that you have a problem...
I am not gonna kid you. You have one Hell of a battle ahead of you!!!
But, realize that this is something that you CAN and WILL overcome!
Read this story:
That guy Bob weighed 120lbs at four years old!!! Three years ago he weighed 670 and now he weighs in at 270! He lost 400 lbs in three short years! If he can do it... So can you!
Start small. This is going to take a while. And, it will turn into a lifetime commitment.
Eating right is very important. But, it isn't enough... Workout. Don't start out trying to workout until you puke or you will give up soon! Take it easy and work up to things. For instance, Kyubi suggested running. This is a great suggestion, but obviously at 670 you can not run. PERIOD. So, start by walking. It may only be a couple of yards at first. I am sure at your weight that walking a very short distance is hard to do. So, walk short distances 10 times a day just to start strengthening your tendons, heart, and lungs! Soon, you will be able to walk further... When this is true, stop parking in the handicapped spots and walk a bit further to your destinations. Soon, take the stairs to go up a floor instead of the elevator or escalator! Before you know it, you will have lost 100 lbs and be able to walk for longer distances.
Start NOW and do not stop. Stick to this goal every single day for the rest of your life (which will be considerably longer if you do!!!)
Good luck - and realize that we are all here for you 110%
-Rip