Before you can begin to start developing your product you need to come up with one. It may sound easy at first but it can actually take a lot of work. Between having so many different ideas and features to go along with them you can easily create a bloated project that is of no value.
There is that word again, value. so how do you create value for something that does not exist? What is value? and How do I calculate it?
Well to answer even that you need to decide what type of project your making, whether it is an app in the app-store, a video game, hardware, the restoration of a car, Choosing your type also depends on value. In this case is it valuable enough for you to spend time working on this project.
What is the end goal for the project? Is it money? Knowledge? A portfolio? Depending what your end goal is for the project you align your value with that. From there you need to ensure your project matches your end goal and not the other way around this is the time to ensure you choose what is best for you or your company.
Remember the time you put into a project is a cost, just like the cost of paying any third party for services, etc.
Okay, you have your idea, kind of now you need to come up with what your project will look like. Whatever you do, do NOT create a "design document", do not spend hours upon hours writing specifications. What you do need to do is envision your idea, write down everything as it pertains from a the consumer of the product. You should not do any "implementation" design. Write down what the end result would be. If it is a game write down the game play, explain how the game is played, whats the player's goal, the environment, the setting, write down as if you are playing the game right now. But most of all write down the core game component. Do the same if it is an app, find the core component that makes the app worth using. Build up your idea around the core component. Push out implementation details until the team is developing, they are the ones to solve that. Yes even if it is you doing the implementation later, In this case "Procrastination is good".
One other thing to mention, be jazzed about your project, if you are not excited for the project and want to do it, then i doubt it will get done, once you "commit" to this it can not be stopped, you should never passed the imagine stage if you are not willing to put a year's worth of time into it. it may take a few months to complete it, but there is still supporting it after. But you should always have a reason for it to be completed and stick with it!
You should also time box your imagine stage, put something together that is complete at the end or even several "time box sessions", but limit them. Too much imagining will cause you to stay in this stage too long and add too many features that will just ensure your project is never done. Once you have your Product and "idea" can be explained and shown to others you are ready for the Planning stage.