The toolbar is where you'll find most of the tools you can use to draw your pixelart.
The toolbar is represented by a mug with brushes and can be found at the left of the screen.
Hit it to reveal all the tools under your disposal:
There's currently about 21 tools in Pixly, if not all of them fit on your screen (which is likely to happen), you can scroll the list up and down too see what's offscreen.
Not all of the tools are in the main toolbar one example of such is the Marquee (selection) tools, you can tap such parent tool to find out a second smaller toolbar by the side of the bigger one.