One of the many things I love about Mac OS X is that I can customize so much of its User Interface (UI) to suit my needs and preferences. The Finder is certainly no exception. I'll show you a simple trick that will help to simplify the Sidebar of the Finder to show just the items that you need (and want) to see. Nothing more. Nothing less.
First, let's open the Finder to explore what's visible by default. Yours should look something like this:
As you can see, there's a lot going on on the Sidebar (the area on the left side of the Finder window), including local files, connected devices, shared resources, etc. In some cases, it's nice to see all that your local computer and network have to offer, but in most cases, we don't need to see all of these options. There are two ways to slim things down a little.
Temporarily Hide Sections
If you move your mouse near one of the section headers, a "Hide" option will appear for that section, as seen here next to the "Shared" section:
By clicking that "Hide" button, you will temporarily hide that section from your view. To unhide, do the same, only you will choose "Show" to show the section. Simple AND effective.
Permanently Hide Items
If you know you won't need to see various items on your Finder Sidebar, your best bet is to go ahead and turn those items off from your view. To do this, open Finder then go to Finder | Preferences... (or press Command + comma on your keyboard). From here, you will see a list of the items available for viewing on your Finder Sidebar.
Simply select or deselect the items you want to show or hide permanently on your Sidebar, customizing it for your needs. Remember, you can always go back into Finder | Preferences... to unhide or hide additional items later on.