Welcome to SparkPeople! Four pieces of advice from my own experience:
1. Make 1-3 small changes each week and build on them. Many times people try to make a bunch of changes or some very big changes right away, and when they aren't successful with those changes, they get frustrated and lose hope. You didn't get to where you are overnight. Don't expect to get fit and healthy overnight either!
2. Long term goals like, "I want to lose 50 pounds" are great, but you need to set intermediate (monthly) and short-term (weekly or daily) goals. When you establish these goals, also set up a system to reward yourself for achieving your goals.
3. Be active in the community. Join SparkTeams and post in the message boards. Blog regularly. Seek out and add new friends. The SparkPeople community is wonderfully supportive and encouraging. If it wasn't for the community, I'd have given up hope and quit a long time ago.
4. Reward yourself for successes, but do NOT punish yourself for mistakes! EVERYONE here has eaten too much, or skipped their scheduled exercises. Forget about it and move on. You can't change what you've done, but you can keep trying to do better. None of us are perfect. Strive for progress, not perfection!
You should also check out the Mind Over Body series of articles, if you haven't already. I think these should be required reading and the very first thing that you see when you join SparkPeople!