Here's a short video on making a white sauce:
http://www.youtube.com/watch?v=5yrbsbQGjRM
Also, mangos here are little like avocados. I let mine sit a few days (not in the fridge) b/f using. I like the Asian ones better--they are small and golden and soften slightly when they are ripe.
Good for you though for giving it a try. Sauces can be difficult and it's a bummer when your hard work doesn't pay off. (I also keep a pizza in the freezer for when this happens.)
A basic cooking class might be worthwhile. Personally, I'd start with a knife skills class b/c so much of cooking starts there.