Hello Grant,
Well, the quickest way to hide completed tasks is to click on the icon which looks like an eye with a tick in it:
To create your own views, make sure you can see the Views / Filters pane on the left (in MLO desktop version) - Alt+F1 to open that pane if it's not current visible.
Then click on the "^Filter" button at the bottom to expand the filters into view and you can set whatever filter criteria you want - It's especially worth learning how the Advanced Filters work.
You can save a filter to change a View or to create a completely new View, using the button at the bottom of the pane.
In iOS and Android you can also create your own filters. In the iOS version, you have to swipe from left to access settings and find the button for editing and creating filters.
I hope some of that helps,
Stéphane