How should I clean out a used fish tank before putting fish or other animals in it?

Wash them well with a non-toxic soap and then set them out in the sunshine for a day which is a natural disinfectant.

By in large, things that will be harmful to your new pets died when the water was drained. As @Coloma said though, just wash it with a non-toxic soap (hand soap works well) and rinse really well.

Non-toxic soap and a salt scrub then rinse it two or three times after leaving in the sun for a couple of days.

Soap ?! Be extremely sure you have all the soap off it completely if you use soap.
Few things can kill fish as fast as soap.

A salt scrub is usually sufficient.
The sun idea is excellent

If you have to use soap maybe fill with water that you will throw out after your sun-soak. Then get your ecosystem going, some plants and late some snail maybe, before you introduce some fish.

DONT USE SOAP.
Vinegar is the way to go, safe for the fish and works great at cleaning up caked on algae and similar muck.

Meh… I’ve been maintaining and breeding fish for years in tanks up to about 100 gallons, and have never had an issue cleaning with gentle hand soap (non-anti-bacterial). Rinse well obviously, but never been an issue.