As some have indicated, it all depends on where you are looking, you haven't said what app or part of the system you were in.
- If you are on the Start Screen, you can always return to the top of the screen by tapping Start.
- If you are in the apps list, it very much depends on how many apps you have installed. If there are more than about 50 apps, WP automatically adds an alpha shortcut list. You simply tap a letter and then select 'A' or in some cases '#' and you can quickly jump back to the top. In practise, it's just as quick to tap Start and then swipe to the Right again.
- If you're viewing a website, then there's no native way that I know that will allow you to scroll back to the top.