If you are interested in forging knives, there is a very good resource on iforgeiron.com. There is a bladesmith named Steve Sells, and he has posted some excellent notes that cover the basics up to some advanced details. Good knife steels tend to have a fairly narrow forging range. Work them too hot and they will crumble; work them too cold and they will crack. There is an old curmudgeon who recommends only working with high carbon steel so you will know the restrictions from the very beginning. I would not recommend this, but I do see his point. He says if you work mild steel, you will develop bad habits. I agree that this is a good point, but there is a huge world out there, and mild steel (as well as wrought iron) are a large part of it, even if not for making knives. Old beat up garage sale files work well if they are solid high carbon steel. Some are case hardened, which you can spot easily with a spark test. Or, if you are an expert blacksmith, they will feel different under the hammer. I once gave a beginner an old file to make a knife out of. He ruined the whole thing. He said, hit it red and it cracks, one spark and it's done. That's about right.