When you feed an active starter flour, it'll bubble up pretty aggressively, so it's good to keep it in a container that has some headroom to allow for that.
Now that you have a starter, what do you do? First, consider your container. When you feed an active starter flour, it'll bubble up pretty aggressively, so it's good to keep it in a container that has some headroom to allow for that. I keep mine in a wide-mouth 24-ounce Mason jar because that's what I have around. The clear glass is nice because you can see the starter rise and fall and easily check what it's up to.