Really simple.
You want plain flour, butter and some liquid. For this I would use milk.
As you are new to this I suggest putting the three things in dishes / a jug.
You want about the same amount of flour as butter, for a small portion 25g of each works well.
Get a sauce pan, put the butter in and start to melt. Try not to be in too much of a hurry, you want melted butter not burned butter.
Once the butter is liquid get a wooden spoon and tip in the flour, and start stirring, and keep stirring.
It will look like a gloopy mess, keep stirring and add a little liquid, stir until it is incorporated and then add more, keep stirring until all the liquid is in the pan.
You now have a sauce. If you cook it longer it will thicken, and it tends to do that quickly so keep stirring.
You can use this for hundreds of sauces, the flour and butter part is the roux.
You might want to add grated cheese tot he milk for a cheese sauce.
For a mushroom sauce I would fry mushrooms in the butter first and then add the flour.