What materials can I use to make a cast for melting lead?
And how would I make the cast?
Most household pans work fine for melting lead. Make sure they are thick enough, don't have plastic parts, and don't have Teflon or similar coatings (Teflon and lead share their melting points, apparently). It's best to use a metal handle (I've used those aluminum camping pan grippers in the past, which are very practical, but I only melted relatively small batches).
Cast iron and steel pans and pots are all okay.
The Dutch oven and a cut-in-half propane tank seem to be fan favourites for melting lead.
Lead melts at the relatively low 327°C (621°F). High heat settings on stoves can reach up to 343°C (650°F). This means all household pans should be up to the task.
You can use aluminum bakeware for casts (like muffin pans, or those disposable oven dishes which can be shaped easily by hand if you don't care too much for detail).
You're probably aware of this, but please be careful when melting lead: the toxic gases can lead (heh) to lead poisoning. Preferably work outside and definitely use a respirator (rated N-100, R-100, or P-100/HEPA; and opt for a reusable one ('half face') as they have superior face seal), eye protection, and protective clothing (in case of splashes).
By "cast," think they are trying to say mold. The beginning technology would be silica sand with a binder. Silica is common beige beach or "play" sand; the binder could be water (damp sand), but will have no strength when it dries. I am thinking something like white wood glue diluted with water. Or, look up "how to make a simple sand mold" on the net. As answered, any kitchen pan could work, but I would not use aluminum because its strength is seriously compromised before 650 F. Industrial lead pots are cast iron (no coatings); I think that is a good hint. Although more expensive, I suggest pewter (mostly tin) or a solder like 50/50 lead/tin for beginning efforts. This is not rocket science; before video games, most kids I knew melted lead toy soldiers for something or other using a kitchen pan.