In other words, "Where do I start?" The best place to start is to understand where you are supposed to finish. What is this project supposed to accomplish that will positively benefit the customers of the project and your organization as well? Watch out for diving with your team right into a discussion of detailed features, for example; that is not
the best way to start! First you should come a full understanding of the project context and its reason for being in the first place.
You may not have been given full insights to this when someone handed you this project, and you may not be able to completely define it on your own. But you need to begin by developing a personal understanding of the project's objectives and deliverables. Start with asking the individual who assigned you to the project, "Why are we doing this project?" Figure out who the customers of the project are, and people who have a stake in the outcome in any way ("stakeholders") and ask them as well. By getting the perspective and expectations of these individuals, you can draft an initial understanding of the goals of the project, what the end product will look like, and what groups need to participate.
Then and only then will you be able to determine who should be on your team—all the functional groups that need to participate in the project. Armed with your understanding of the project's objectives, communicate to the functional managers what you and the project have been asked to do—they have to see why they should give resources to a new effort!—and then to new team members as they come on board.
With those people brought in as your initial core project team, the next step is to mature together that vision of the project's goals. Initiation stage activities focus a core set of cross-functional team members on creating a Project Charter, an expression of what the project is to do. By the end of Initiation, the team fully understands, and the executives have approved, "what we've all agreed to do." This early vision work is the critical foundation of moving on to the Planning stage, where the team will define in detail the project scope and develop the project plan.
Take a look at the Project Charter document for an idea of how project goals should be expressed early on. Another approach to this is our Project Vision document, which helps the team focus on understanding the customers of the project and the benefit the team is supposed to deliver, before the team dives into talking about more specific scope and features.
The Project Phases section of the New to Project Management Fast Track provides a step-by-step path through the early day of a project, to help you get going on the right foot and see how initiation work then proceeds into defining and planning the project in detail.