1) Finger Test
How much room should you have in your shoes?
Slide your index finger between your heel and the heel of your shoes. Your finger should fit snugly, but not too tight or loose. If it is tight, chances are you need a bigger size, and if it is loose, chances are you need a small size.
2) Walk around in the shoes
To check the fit and comfort. Ensure they don't rub anywhere. Your heel should not slip or slide while walking. Try them on a carpet so it doesn't scratch or damage the shoes in case you need to exchange them.
3) Check the space at the tip of your shoes
Stand up and ensure there is about 1.5 cm (about the width of your fingers) between your longest toe and the tip of the shoes. The longest toe, can sometime be the second toe. Your toes need ample space to allow your toes to wiggle, so that you do not get calluses.
(An illustration of how your feet should fit in the shoes)