This is pretty much dead simple. Cheese Sauce of the Technological Gods!  
 
2 tbsp. butter (unsalted sweet cream of course)  
2 tbsp AP (All Purpose) flour  
1 cup warm milk (meaning warm the milk up now as you're reading)  
1 cup of your favoritest shredded cheese. Cheddar is the usual kind for this.  
0.5 tsp salt - you can skip the salt, but it's not going to taste like much if you do.  
 
Prep time is about 5-8 minutes, yields about 2 cups of cheesy goodness.  
 
Directions:  
 
In a 1 quart (I used a 2 quart) glass microwave safe bowl, melt the butter on high about 0:30 to 1:30 - the idea is to get the butter hot, not just melty looking. Remove from microwave (using an oven mit as most likely the bowl is hot!) and add in the flour one tbsp at a time, whisking the whole time until it's nice and smooth. If it clumps up you're fine, so long as the flour is all mixed in. Add the milk, still whisking the whole time. Now put the bowl back into the microwave for 2:00 - 3:00 on high, removing every minute to stir things around.  
 
At this point it should be pretty hot, to the point you're using an oven mit because you need to. Seriously, if you don't have one go get one because you'll need it. Add the cheese and salt, stir it around (gently) until the cheese is all melted in and the sauce looks even colored.  
 
Now is the time to doll it up too - cayenne pepper, garlic powder, onion powder, etc. Add salsa and hot sauce if you're Substandard aka Ss.  
 
Pour over chips to make instant nachos without those crusty nastified edges you'd get microwaving them, or pour over broccoli if you're one of those vegetabletarian weirdos. ;)  
 
If you really like cheese, add 2 cups instead of 1 - but be forewarned, it's going to be a *very* potent sauce!