By the time you have finished this tutorial you'll have learnt how to make a shape with both hard and rounded corners.
1. Open a new canvas and make it 200px in both width and height.
2. Select the Rectangle tool in your Tool's palette and make a shape 100px width and 100px height. Rasterize it by right clicking on the layer and select 'rasterize layer'.
3. Select the 'Elliptical marquee Tool'
4. Use fixed dimensions, 50 px width and 50px height. Click and move the circle so it touches the left and upper border of your shape.
5. Press Ctrl+Shift+I, to invert your selection, which we want to remove. As you can see the upper left corner is now selected, but so is the major part of the rectangle, which we dont want to remove! In order to fix this we need to deselect this are, by removing part of the selection.
6. Select the Rectangular Marquee Tool. Drag a shape while holding Alt. Drag it from the top right corner to the lower left, but dont go over the top-left area we want to remove!
It should look like this:
7. Now repeat step6 for the lower left part, and all thats left is a selection that removes your top left corner! Press Delete.
8. Now repeat step 3 till 7 for the other corner, and you should have this end-result:

Simple as that!