Coal

Coal is a vanilla minecraft item that can currently only be obtained through mining and by running a mass-fabricator

Coal is a very useful fuel for it is plentiful and cooks 8 items per coal piece, but can be used more efficiently by turning it into coal coke. You can also turn coal into diamonds through a somewhat long process that requires about a stack. Coal is a non-renewable resource, at least until you get a mass-fabricator. For these reasons, it may be wiser to use renewable charcoal for your fuel needs instead.