If you are buying a brand new desk for your machine, there ane many different types available - choose one to suit your budget and decor. The type which are wider at one end than the other are well worth considering if the desk is not exclusively for computer work. It is important not to obstruct this grille or any other vents which allow cool air to enter the case. For similar reasons, always keep the system box the way up it was designed to be. Don't stand desktop boxes on their side.
Heat rises, and if the box is the wrong way up, it can build up in unwanted places.
Many computer users ignore that most essential items of equipment - the chair. Do make sure you use one that is comfortable and provides plenty of support. Most office supply shops will have a selection and will advise you, but do try before you buy. Make sure you know how to do all the various adjustments. Once again, there are many different types available.