plane, select - whenever you open a new sketch, you're in the "triclinic" or 3-plane view by default. Normally you want to start in something like the X-Y plane. Sometimes, when you first start by selecting something like a line or circle, you're presented with all 3 planes and you can simply click on a plane. But often it stays "slanted". But if, before you ever select on an item, you click on one of the arrows on the "triclinic" symbol in lower left, you can "collapse" that dimension so you start off in just 2 dimensions.