Hi fluffyfeet,
For a basic roux sauce, melt approx 75-100g butter with approx 2 tbsp plain flour. I also add 1/2 tsp of mustard powder to this.
Slowly is the key, let it all meld together keep stirring and let it slightly bubble.
You then gradually add milk, 1 pint, and stir stir stir. It will warm and thicken over about 10 mins. Don't use a high heat because you may find it sticking to the bottom of the pan and it'll go lumpy.
The milk could be warmed with an onion, bay leaves, nutmeg and pepper and then added to the butter/flour or just add milk from the fridge.
Once the white sauce is thick, add grated cheese. Cheddar, red leicsteshire or cheese you like till melted.
You can fry leeks and bacon to add to this or mushrooms.
I make a basic white sauce and add shredded crab sticks for an omelette filling sometimes.
Happy cooking :D