As Doug said, revolving a half circle. (the tutorial was made in Inventor pro 2011 but it applies to all versions of Inventor
Next in the sketch environment draw a half circle. This can be done in various ways that all give the same result. In my case I draw a line and then by activating the line command again and holding the left mouse button I dragged one edge of the line to the other end. You could also draw a circle then a line from top to bottom (or left to right) and then trim the one half. Another way is to draw the line and then a 3 point arc using the center of the line as the arc's center and the two ends of the line as the other input points.
When you are done click finish sketch.
Then click Revolve in the Model Tab
The profile to be revolved should be automatically selected. If not click inside the half circle. Then you should choose an axis for the profile to be revolved.
Click on the line to select it as an axis and a preview of the revolved solid is displayed. By default full revolve is selected. You can choose an angle revolve or to another surface instead of full revolve
click ok and you are done!