If you are talking about the squarish storage tubs, those are not going to work well at all. THe sides are usually too weak, you will need to build something around it for support if you are going to do that.
For an outdoor pond, you can purchase stock tanks made by Rubbermaid which are used to water cattle and other livestock. You can find those at feed stores, hardware stores, Tractor Supply, and some home improvement stores may have them. THe smallest one made is 50 gallons I believe.
If you dont want to do that then try to find a round container. Square or rectangle just isnt as sturdy.
You will need a filter for it. I am building my own and this is the one I am making. Please take time to read it, its a good filter.
http://www.skippysstuff.com/biofiltr.htmIf you read this and cant understand it I can try to explain it to you, but this is pretty easy.
After you have a filter all you need is the pump. Maybe toss in some plants and your done!