From your main calendar or task view, tap the menu bars at the bottom right and choose tag filter.  

At the top of that entry at the 'mode' option choose if you want to filter by : OR, AND, or NOT.

Then check or uncheck the tags listed to set your filter.

You can then tap the menu bars at the top right of this page and choose to create a new tag, save this filter, apply a saved filter, check all tags, or uncheck all tags.

You can see when a tag filter is applied in your calendar or task view by looking at the bottom of that view. If you see a little tag icon, that means your data is being filtered by that tag.