I make this all the time:
Boil whatever pasta you choose to al dente (follow times on box).
While the water is coming to a boil and your pasta is cooking, melt two tablespoons of butter in a pan, add two tablespoons of flour, stir until well combined and let it cook on medium - low for a minute or two.
Slowly add about two cups of milk (more or less, let it bubble up in between, until you have your desired consistency for the sauce). You can sub some of the milk with chicken stock, tomato juice, V8 to change things up. If I have some on hand I will also add pureed veggies like squash or carrots.
When the sauce has a nice silky consistency add grated cheese over low heat/simmer. You can add as much or as little as you like. I go with one cup of grated cheddar and a 1/4 cup of parmesan, but in a pinch I have even used Kraft Singles or just sliced deli cheese. Let the cheese melt and combine with the cooked and drained pasta. Season to taste with salt, pepper and (optional) nutmeg.
This takes about 15 minutes to prep/cook.