To export your work, either choose File › Export… from the menu, or click the Export button in the toolbar. Sketch’s canvas is infinite in size. This means that when you want to export your drawing you have to tell Sketch which part(s) you’re interested in.
In Sketch 3 we’ve greatly improved the exporting workflow. Read more about this in Exporting Layers.
When you click the Export button in the toolbar, Sketch will pop down a sheet listing all exportable layers in the canvas; Artboards, Slices and regular layers. You can export some or all of these from there. Note that if you have some exportable layers selected before you go to Export, Sketch will by default only export those.