The easiest way to use tabs (of all kinds) in WP8 is to use the ruler bar.
If the ruler bar is not displayed, click View then Ruler.
With the ruler bar displayed, you can remove existing tabs, if desired, by individually dragging them off of the ruler bar and dropping them. You can also clear all of the existing tabs by right-clicking in the narrow white space below the ruler where the triangles appear, then choose Clear All Tabs.
To set up a decimal tab (or any other type for that matter, right click in that same narrow white space and choose the desired tab type. Then decide where to place the tab and click on the ruler bar at that point to set it. When you create subsequent tab stops, the tab type remains the same until you change it or exit.
One more item. Changing tabs only affects stuff that come after the change and before any subsequent tab changes in your document. In other words, pay attention to where the cursor is in your document when you change tabs. The change only affects stuff from the position of the cursor downward. If you want the same tabs set up for your whole document, be sure to be at the top of the document (with the cursor, not just scrolling up there) when you make the changes.
BlackburnKL