Assuming that you have Winzip or StuffIt installed on your computer, you can access the files archived inside a .zip or .sit file by simply double-clicking the archive (a file ending in a .zip or .sit extension). Double-clicking one of these kinds of files will open up a window that displays the contents of the archive. In most cases, you can double click a file inside this window to use it, or you can select it and drag the file to a folder to view later.

Depending on how you elected to install Winzip or StuffIt, you may be able to right-click a .zip or .sit file and have the program extract its contents into a new folder for you.