A few things to keep in mind if you go this route: 1) gas costs - is it worth driving around trying to find extra metal ? steel is roughly 12$ / 100 lbs. Most of the stuff you find (grills, exercise equipment, furniture) is made out of this. 2) transportation - easy if you have a big truck or van, but not so easy if you have a car. 3) location of the scrap metal yard - how far away do you have to drive to cash in ? Ties in with #1. 4) storage - where to keep accumulated stuff. Easy if you have a big yard and neighbors who aren't nosy, but all this stuff stacks up if you dont make trips immediately after finding something.
If you find the stuff at work and are driving home anyway none of this stuff really matters, but otherwise its time / money tradeoffs. Give it a shot though - learning about metals and trading markets is cool and you start thinking about what stuff is actually made of that you take for granted every day.