Do US citizens need a visa to travel to Australia?

I'm going to australia for 2 weeks to attend my daughter's marriage.

Yes.

Yes you do check out this website it will tell you how to apply
embassy.australia.visabureau.com

good luck

yes you need a visa to visit Australia

You do need one, and the best Visa for you would probably be the:
ETA (Visitor) (Subclass 976)

This visa is designed for people who are outside Australia and want to visit Australia for holidays, tourism, recreation, and informal studies or training.

It should cost you about A$20. Apply at LEAST 2 weeks before you travel, but preferably sooner.