First of all you will need to decide what style and size of computer casing you will be purchasing. Your options are either a tower casing or a design top casing design. Most computer casing today will come as default tower casing as they take up much less space compared to the desktop. By taking a tower you can save at least 50% space on your desk.
Once you have chosen the tower casing you will need to consider whether you want a small mini, medium midi or the larger full tower.