Tap the Layers icon on the canvas to open your Layers menu. You'll see the current active Layer is highlighted - this layer is what you are currently drawing in. You can tap any Layer to activate that layer instead.
Concepts has two Layering modes - Automatic (this sorts your strokes for you by tool type), and Manual (this gives you manual control over which layer you draw in). To toggle between the modes, tap the Sorting button at the top of the Layers menu.
By default, Concepts uses Automatic layering to allow for faster editing and organization, without you worrying about layers before drawing. You can read more about this in our How does Concepts decide which layer to draw to? faq. To switch to Manual mode, you can simply tap any layer to draw in.
For an illustrated tutorial on layers, how they work, and how to use them to organize your sketching, check out How to Use Layers to Take Notes, Illustrate and Design.