It's too big a question involving multiple shapes and layers. Have a play with the Shape layers. These are what are called 'vector' objects, and _much_ better to use when making graphics like this.
The cloud is a good place to start, and is made of three circles (red) and one rounded rectangle (green)

When you use the rounded rectangle you need to adjust the radius in the Properties panel (red highlight). You can just drag across one of the radii to adjust all four so long as the little chain link icon is switched on in the middle of the four radii.

Have a go with that, and come back when you get stuck.
Ahhh I need to tell you that when using Shape layers, you need to set fill and stroke colours.

If you want either a stroke or fill to be turned off, use the white icon with red bar through it.