The most obvious requirement for blacksmithing is, of course, a metal to work on. Being that there are so many metals out there, it may be rather difficult to decide what to start with.
When researching the subject, I found many people asking if alluminum was a good material for beginning blacksmiths. I later discovered that this is not only a bad idea, but can be life threatening. When ground into powder, alluminum becomes toxic. Brass is usually easy to work with, but the most recommended metal for beginners is, unsurprisingly, iron. Iron is, after all, so useful that it is still used today, albeit normally in the form of steel. Rail road spikes are popular among beginners because they are relatively easy to work with and cost about a dollar each. Certainly an easy and cheap place to start.