If you have time, chop a small onion, a carrot and a couple of celery stalks. Add these to a pint of milk, along with a few peppercorns and bayleaves and bring the milk to the boil. When it starts to bubble, turn the heat off, let it cool and strain off the veg.
Melt a tablespoon of butter in a saucepan and then add a tablespoon of flour (white for a smooth sauce, wholewheat for an interesting sauce). Stir the flour into the butter for a few minutes over a low heat, then add some of the strained milk. Whisk gently and gradually add more of the milk - as it thickens up, just add more milk. Eventually you will end up with a pint or so of a good *bechamel* sauce - just add loads of grated strong cheddar (at least 6 oz) and stir well for a fabby cheese sauce. I often use several types of cheese in my sauces - a mix of extra strong mature, smoked applewood and parmesan or gruyere is great !