Form General Instruction of the Roman Missal (319-324):
Following the example of Christ, the Church has always used bread and wine with water to celebrate the Lord’s Supper.
The bread for celebrating the Eucharist must be made only from wheat, must be recently baked, and, according to the ancient tradition
of the Latin Church, must be unleavened.
The meaning of the sign demands that the material for the Eucharistic celebration truly have the appearance of food. It is
therefore expedient that the Eucharistic bread, even though unleavened
and baked in the traditional shape, be made in such a way that the
priest at Mass with a congregation is able in practice to break it
into parts for distribution to at least some of the faithful. Small
hosts are, however, in no way ruled out when the number of those
receiving Holy Communion or other pastoral needs require it. The
action of the fraction or breaking of bread, which gave its name to
the Eucharist in apostolic times, will bring out more clearly the
force and importance of the sign of unity of all in the one bread, and
of the sign of charity by the fact that the one bread is distributed
among the brothers and sisters.
The wine for the Eucharistic celebration must be from the fruit of the grapevine (cf. Lk 22:18), natural, and unadulterated, that is,
without admixture of extraneous substances.
Diligent care should be taken to ensure that the bread and wine intended for the Eucharist are kept in a perfect state of
conservation: that is, that the wine does not turn to vinegar nor the
bread spoil or become too hard to be broken easily.
If the priest notices after the consecration or as he receives Communion that not wine but only water was poured into the chalice, he
pours the water into some container, then pours wine with water into
the chalice and consecrates it. He says only the part of the
institution narrative related to the consecration of the chalice,
without being obliged to consecrate the bread again.
Jesus celebrated his Last Supper with bread and wine, so bread and wine is necessary. There is not a consensus between Western and Eastern Christianity whether the bread should be leavened or not. This is based on accounts different evangelist and of Paul whether the original Last Supper of Jesus was a Passover supper with unleavened breads or not; due to this uncertainty, both variants are OK.
About wine, I've heard that some of early churches celebrated Eucharist with unfermented grape juice, so it could be theoretically possible to substitute this for wine. But lager is definitelly not suitable.
Addendum: in bad conditions like a prison (many priests are still being arested in some Christianity unfriendly, usually Islamic or communist countries), any bakery suits for "bread" and even a raisin smashed in water can qualify as "wine". No priest would approve to celebrate mass with this instead of standard bread and wine if he can choose, but such exceptions are possible in really harsch conditions.
And an anecdote for the end: I don't know where this happened, but once a drunken priest came to a baker's shop and started to pray a eucharistic prayer. Then he had to buy everything in the shop and destroy it with piety, because nobody could be sure what is ordinary bakery and what is Body of Christ, and risking desecration of Eucharist was not an option.
