How do I know if I need a trust or a foundation?

Talk to a lawyer... because you aren't creating either of those on your own. They can explain based on your personal circumstances which is better. There is no one right answer.

A foundation is an organization set up to give money away. So I'm not sure that's what you want. A little more information about what you want to accomplish will help us answer you, but yes, you would not set up either without the aid of an attorney to get it right.