There's no law preventing an undocumented (illegal) immigrant from marrying a U.S. citizen, but getting a green card (permanent residence) is not quite so simple.
U.S. citizens with foreign-born brothers or sisters may obtain green cards (lawful permanent residence) for them and their spouses and children, but only after a long wait.
If you're facing removal from the U.S. (deportation) and have no means of defense, you may request voluntary departure to avoid some future immigration consequences.