Look through the dozen or so other similar posts about 'where can I find steel' ,etc.
I wouldn't go to a scrapyard, as they probably just paid someone for that steel, and don't usually sell it back. Just go to a steel yard.
My personal favorite is Pontiac Steel. Its about 50 cents a pound for cutoffs on the shorter racks. Then they have a garbage bin with small chunks that are free. Go in, buy a few pieces off the short rack, then ask for some small scraps for welding.
If you are learning to weld and fabricate, then the best thing is having a lot of steel around. If its saturday night and you are working on the truck, you don't want to not have some material for what you need.
In my garage, I usually have on hand:
1.5" square - 0.120 wall
2.0" square - 0.250 wall
1.0" sqaure - 0.080 wall
1-1/4" sch 40 pipe
2-1/2" sch 40 pipe (for driveshafts)
2" x 0.250" angle
1.5" x 0.120" angle
1" x 0.125" bar stock
2" x .375" bar stock (good for making shackles)
6" x 0.5" plate
3" x 1" bar stock (those last two are usually for recovery points)
and a big bin of all the scraps from the years.
Now I need to build up stock of solid rounds for my lathe..