The reaction between aluminium and steam produces aluminium oxide and hydrogen gas.
However, this reaction does not always take place because aluminium is usually coated by a thin layer of aluminium oxide. This is also the reason for why, when it happens, the reaction is very slow.
Anyway, the balanced chemical equation for this reaction looks like this
The hydrogen gas is usually collected over water.
Notice that the reaction produces aluminium (III) oxide, the most common oxide formed by aluminium. This is also the oxide that forms the thin layer present on the surface of the metal before the raction takes place.
SIDE NOTE I'm not sure what you mean by "work out" the chemical formula, since no additional data is given, so if this is not what you're looking for, let me know, I'll edit my answer accordingly.
You can figure out the actual formula of the oxide, but you're going to need some information on how much aluminium and water reacted.