The first questions you should ask yourself are:
If you are already a team working on the project, it is highly important that every team member has the same understanding of the actual purpose. When you have a shared understanding of the purpose, write it down. Then you can return to it later in the process if it gets difficult to keep focus.
In order to properly define the purpose of your project, you should understand and map the decision-making process in which you wish the young people to intervene. It will be easier to formulate a spot-on eParticipation project if you know where exactly the decision-makers are in the process. It is usually not under your control, but try to intervene as early as possible!