Hoogle's approach is indeed excellent, but as alternative, here's a different approach that doesn't take any experience painting with brushes in PS.
1. Select the sky and, since you are just starting out in PS, to be safe, save the selection, and give it a name, eg,"sky". There are lots of different ways to select an area, and deciding which selection method to use could, itself, occupy a different thread. FWIW, I used the "quick selection tool".
2. Hit cntrl-J to put just the sky on it's own layer
3. Re-select the sky and add a layer mask to the sky layer by clicking on the third icon from the left at the bottom of the "Layers" palette.
4. Click on the icon representing the sky in the layer you just created. This will select it for future operations.
5. Go to the pull down menus and select filter / blur / motion blur. Set the angle to be 0 degrees (ie, a horizontal blur), and the blur distance to be around 300 pixels. Hit OK.
6. You're more or less done.
The above steps may look complicated the first time you do them, but it took me about 5 minutes to write up the description, but only about 15 seconds actually do all of the above work.
There are some other small tweaks that will slightly improve the above, and if you like this method, I'll be happy to describe them as well... just let me know.
Below is a screen grab of the really complicated (LOL) layer stack, and then the final result.
HTH,
Tom M