Learn French: Il y a un canard sur le lac. - There is a duck on the lake.

No, the -aux ending is for words ending in -al (like un cheval becoming des chevaux). Because lac ends in -c, it follows the standard rule. You just add an -s to make it lacs.

How would I make this whole sentence plural? Do I need to change il y a?

You would say Il y a des canards sur les lacs. The handy phrase il y a means both 'there is' and 'there are,' so it never changes for plurals. You only change un to des and le to les, and add an -s to the nouns.

Do I pronounce the d at the end of canard?

No, the final d in canard is silent, which is very common for consonants at the end of French words. You just pronounce it like 'ca-nar'.

Why do we use sur instead of dans for the lake?

In French, just like in English, we use sur (on) because ducks float on the surface of the water. If you were talking about a fish swimming underwater, you would use dans (in) to say dans le lac.