From reading your message it looks like you intended to post and example image. It didn't post. After you preview your message you have to link up the picture again.
Anyway, here are some basics
1. In the layers palette, the second button from the left at the bottom is the one that adds a layer mask. Just click it and you will see a white rectangle show up on the layer. You can select the mask at any time by clicking on its icon. To deselect it, click the picture icon. To delete the mask, right click and select the delete option.
2. Making sure the mask is active, paint right on the picture with black paint. It will actually be painting on the mask, and you can confirm this by looking over at the mask icon. You will also see the under image showing through. If you make a mistake, paint with white. Set your brush to partial opacity if you want a partial mask.
Regarding clouds, I'm not sure what you mean. The clouds feature in Photoshop creates faux clouds that are meant for special effects. If you want real clouds in a sky, here are some free ones:
http://art.net/~jeremy/photo/public_texture/index.html