In order to keep your computer clean, you must know what to do when it actually gets dirty with dust, fingerprints, a little coffee or beverage spill and some smudges beyond full description.
Keep a lint-free cloth to clean smooth shiny surfaces and dust off screens, you may slightly dampen the cloth with mildly soapy water with a hint of dish-washing liquid so that it does not drip any water but is slightly moist so that it picks up all the dirt. Do not clean any ports a moist cloth. You can use approved electronics cleaning alcohol for that using a soft brush.
If you cannot clean your computer surfaces and screens this way because of years of dirt and grime accumulated during usage, you should take it to a qualified technician who can disassemble the unit, clean it inside-out using the correct materials and solvents and then carefully reassemble the unit. At TekNerds we do this service for an entire array of different computing equipment and peripherals.
Try not to eat in front of your computer to avoid food particles and liquids getting lodged inside keyboards/mice and smudges or splashes getting onto screens and monitors.