ok this is your BASIC bechamel(feel free to add cheese, red wine...white wine...) at your discretion (bechamel tastes well pretty plain) so add something.
roux(equal parts fat and flour)
melt the fat mix in the flour till thick and bubbly dont make it brown or anything because bechamel is white, if your roux is brown.. start over and use your brown roux for something else like a gravy. If it smells distinctly nutty or like buttered popcorn(if you use butter) then youre good and dont cook your damn roux any more.
for this you need a whisk and I cant stress this enough buy a damn sturdy freakin whisk and its pretty useful for everything. No flimsy crap because you'll just be screaming when your sauce comes out wrong. Theyre not but like $5 maybe $10 at a kitchen store(ie Williams Sonoma, Lechters, your local restaurant supply store, Wal Mart and Target even) and if kept out of the dishwasher and washed and dried asap they last forever.
add/whisk in milk(into the roux) until the mixture is thick like a potato soup.. continue to whisk but do not add more milk and cook the flour.. see how thick it is.. if it is thick enough to coat the back of a spoon this is how you test for the right consistancy.. run your finger in a line down the spoon and if the line stays(the sauce does not fill in the line) youre good...
now add your gruyere(shredded) cheese, or parmesean.. or whatever..add as much as you like, though uh it will make your sauce very thick so beware before you go dumping it all in at once.. add a little at a time
you also need salt(kosher) and pepper(preferably white)
there you go
sauce mornay. total yum on veggies, pasta... etc