Before getting started, make sure you know what you are trying to do.  Just opening up the case and "fiddling" isn't a good idea.

 If you are trying to install a new part, make sure you know if it is compatible with the other parts of your computer.

 If you arereplacing an existing part, make sure the replacement is compatible with the previous part.IE does the graphics card go in the same slot or is it a different kind?  Does the old hard drive have the same connecting cable as the new one?

But if you are trying to figure out what is "making that loud noise" or solve some other problem, then opening up the case and diving in may be your only option.  WARNING: you can totally mess up your computer if you don't know what you are doing.  Just an FYI.  :-)