Need a blank space with no sound at the beginning of my project.

I uploaded my finished project on youtube. Love how it turned out, just wish it could have a little bit of "nothing" before it starts. As soon as you push "play" it starts right away...
Now that my project is done, how do I create a 5 sec space of just nothing (black with no sound) in front of my project?

Insert a gap. Place the playhead to the beginning & press keyboard shortcut Option+w or go to the Edit Menu then Insert Gap.

That's pretty easy: In FCPX, set the playhead to the beginning of your project and hit Option-W. That will put a gap clip in front of everything. You can adjust the length of this gap clip to your liking. (By the way, 5 seconds is quite long. Many of your potential viewers will not have patience enough. Two seconds should be more than enough.)
The gap clip will display the background color of your project. Should you prefer another color, you might as well insert a color generator instead of the gap clip.
In any case, once you're done and re-upload your video, your current YouTube link will break. Should there already be a considerable number of viewers, I'd reconsider the substituition plan