The unbalanced equation for iron combining with sulfur to produce iron sulfide is:
This is not the complete formula for the chemical reaction.
All three of these substances are solids at (our) living temperature. Iron occurs naturally as a compound in rock formations or on the earth's surface as oxides. The iron will need to be reduced to filings for the above reaction to take place. Sulfur appears as a powdery yellow chemical that is "farmed" near some areas of volcanic activity.
When iron and sulfur in filings and powder form are combined they will form a mixture of the two chemicals. This mixture can still be separated using a magnet to pull out the iron filings.
For the chemical reaction to take place, there must be an ignition source or high temperature heat applied to start the reaction process. Once the reaction has started, it is exothermic, and generates a flaming heat. The heat generated is more than enough to continue the reaction so no further external heat is required.
There is a video here:
Warning: Endothermic reactions are dangerous and appropriate care must be taken in their practical investigation.