Making a basic round shape, such a cylinder, is very easy. Simply draw a circle, then use the extrude tool to give it volume:


This is great if you're making the land of smoke stacks, but what if you need to have a dome or bowl shape? Sketchup uses a lathe system. First, Make a 2d representation of what you want your rounded surface to look like. Then place a circle underneath it that will be the diameter of the finished product:

You can do this by using the Arc tool:
Then highlight the area beneath it that you want it to follow, pick the Follow Me and select the 2d surface:


And now we have a domed surface on top of our cylinder!
You can also create more intricate patterns by first making a flat surface, and then creating the design you wanted using the Line and Arc tool. It helps to make certain that the design area is only half of the circle that you will be using:


Then delete anything you don't need:

Finally, use the Follow Me tool to make the shape:

Just delete the circle if you don't need it anymore and you'll have your new, rounded surface ready to go.
Using the Tools on Surface plugin, you can directly manipulate the rounded surfaces. I'll get more into that on the next post.
Happy modeling!