If you want Safari to go directly to the Top Sites when you open it,
Safari Preferences:
Safari opens with: A new window
New windows open with: Top Sites
Ignore the Home Page preference
If you want Safari to go somewhere else (any website you choose) when you open it, navigate to the website that you want to use as your home page and then
Safari Preferences:
Safari opens with: A new window
New windows open with: Home Page
Click the "Set to Current Page" button under Home Page and it will automatically fill in the URL from the page that you navigated to.