french help

I'm doing my oral exercises on the computer and it asks me a series of questions. This is the example:

Vous entendez: Les jeunes gens n'ont pas de famille.
Vous dites: Donnons une famille aux jeunes gens.

And we are required to repeat this basic structure for a list of sentences.

I have some questions though. How do I know when to use a, a la, or au(x)?

Merci beaucoup!!