For the first 20 years of the Mac’s existence, you began your workday by double-clicking the Macintosh HD icon in the upper-right corner of the screen. That’s where you kept your files.
These days, though, you’d be disappointed if you did that. All you’ll find in the Macintosh HD window is a set of folders called Applications, Library, Users, and so on—folders you didn’t put there.
Most of these folders aren’t very useful to you, the Mac’s human companion. They’re there for OS X’s own use—which is why, today, the Macintosh HD icon doesn’t even appear on the screen. (At least not at first; you can choose Finder→Preferences and turn the “Hard disks” checkbox back on, if you really want to.)
Think of your main hard drive window as storage for the operating system itself, which you’ll access only for occasional administrative purposes.
So where is your nest of files, folders, and so on? All of it, everything of yours on this computer, lives in the Home folder. That’s a folder bearing your name (or whatever name you typed in when you installed OS X).
OS X is rife with shortcuts for opening this all-important folder:
Choose Go→Home, or press Shift-⌘-H.
In the Sidebar (Uniwindow vs. Multiwindow), click the icon.
You might also consider adding your Home folder to the Dock (The Dock) or making it the window that appears when you press ⌘-N or choose File→New ...