Hi there and welcome to SparkTeens! Try to look at this as a healthy lifestyle rather than another diet. Work on changing your habits into healthy habits and you won't have to diet on and off. The Resource Center is a great place to read articles, get tips and ideas, see exercise demos, exercise videos, motivation, etc. The message boards and sparkteams are another great place to get inspiration and motivation from other teens that are going through similar things.
Here are some articles that I recommend you take a look at:
For the last, 2 months, been trying to loose weight. i always start off good, and then i stop. I need something to keep me going for the next 10 weeks.