You don't need to worry about utility applications/tweaks. Just shut down every few days and OS X will look after itself. It's a bit of a daunting prospect when you've come from Windows, but just trust that you don't need to do any maintenance at all.
Personally I just use Safari for the Internet browser, it's by far the fastest on OS X, has much better resource optimisation, and the multitouch gestures are far ahead of the competition.
You'll get mixed responses about the mail application. A lot of us use Mac Mail which is fine, but if you have a Gmail account or work with a lot of subfolders, I find there can be a lot of syncing issues with the mail server. If you run into such issues or don't like the built-in mail application, you can try Outlook (which comes with Office 16/365), or for best reliability use Thunderbird -- though it doesn't look pretty, it always works.