Sky

Make your canvas size 800 x 600. Adjust your workspace so you see it all at 100%.

Make the canvas color blue. Do not use the Paint Bucket! Click off the canvas with black arrow and click on the color square in Properties (or go to Modify>Canvas>Canvas Color and set the color there).

Make sure you have a bitmap layer so you can draw...

Set the Brush color to white, Unnatural>Viscous Alien Paint... 

Draw a few clouds in your sky. Name your bitmap layer "clouds".

Insert a new bitmap layer. 

Set the Brush type to Airbrush>Basic, yellow, size 100 - 200. 

Click a few times in the middle of your stage, until you get a sun you're happy with. 

Select your black arrow tool, then you can move the clouds where you want. 

Also move the "sun" layer under the "clouds" layer.