We've all seen the pictures and watched the A&E TV show Hoarders.
Look at this picture for a minute and ask yourself: "Where do you start?"
For many, including the hoarders, this can be overwhelming and can even paralyze your mind to the point where you don't want to do anything.
Writing can be the same way. Let's say you have an epic fantasy in mind. Where do you start?
Just start somewhere.
Just start writing and let the story flow. This works especially well if you are not an outliner. If you are, God help you--just kidding, there's hope for you outliners too. For outliners, just throw down as many ideas as possible. Or use a brainstorming diagram where you may have a central idea and have many things branching out from it. Like this:
Use whatever tools works best for you. But start today. Do it now. Don't get overwhelmed by the percieved scope of the project. Just write it.