There are limited options for dealing with wrong answers on Stack Exchange, especially if you have not earned any reputation points.
Answering (no rep needed)
You have to have reputation before you can do anything?
It definitely can seem that way, but reputation is actually required more for the minor site functions than the major ones. New users can make question or answer posts right away. As TRiG says, if you can post an answer of your own, that would give you a place to discuss the errors in the earlier answer.
Unfortunately, as Kris pointed out in a comment, knowing that an answer contains errors doesn't always imply knowing the correct answer to a question. Using an answer post just to correct another answer is not allowed, so if you can't answer the question, this is not a possible course of action.
The other ways of dealing with bad answers—flagging, comments, and downvoting—all require you to earn some reputation, and the first two are subject to fairly heavy moderation.
Flagging (15 rep needed)
The current flag system allows answers to be flagged for four reasons: "spam", "rude or abusive", "not an answer", or "in need of moderator intervention". None of these is really applicable to a question that is merely incorrect. The "not an answer" flag is sometimes raised on egregiously wrong answers, but the general consensus on Meta Stack Exchange is that this flag should only be used for posts that do not even attempt to answer the question, not for posts that try and fail to answer the question correctly.
(Edit: as Tonepoet's answer mentions, there is also a fifth flag option, "very low quality", that is only available for certain recent answers. Its meaning is a bit unclear, but it also shouldn't be applied to posts that try and fail to answer the question correctly.)
Thus, flagging is rarely the right way to respond to erroneous answers.
Comments (50 rep needed)
Constructive criticism is one of the intended uses of the "Comment Everywhere" privilege, so in principle, you should be able to use comments to point out errors to the author of an answer. However, diamond moderators have the ability to delete comments, and they often will in circumstances where they feel that a comment chain has become a discussion, or if they think that it's unlikely that the post author will respond productively to the comment. A commonly expressed sentiment on Stack Exchange is that comments should be thought of as "ephemeral" and "unimportant": although I don't really agree with applying this model to comments that point out errors, it's still likely that moderators will take this perspective when dealing with your comments. Maintaining a polite tone will reduce the chance of your comments being deleted.
Comments by themselves don't have any effect on the answer, but a comment may serve as a visible note to readers that lets them know about the errors (this is especially true if the comment gets upvoted).
Downvoting (125 rep needed)
Downvoting is the least restricted, and most encouraged way of dealing with incorrect answers. But it also has the highest reputation limit, and each downvote on an answer costs a point of reputation. Voting affects the relative ranking of the answers to a question, and answers with a sufficiently low total score get "grayed out".
The meaning of downvotes is somewhat subjective: the tooltip for downvoting answers merely says "This answer is not useful", while the Help Center says "Voting down, also known as "casting downvotes", is how the community indicates which questions and answers are least useful. [...] Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect."
Downvotes are subject to moderation only in the sense that voting abuse (like "serial downvoting" a particular user's posts, sock-puppetry and the like) is investigated and perpetrators may be disciplined. Aside from these obvious exceptions, it's up to each user to decide whether an answer ought to be downvoted.
A downvote may optionally be accompanied by an explanatory comment. This is never required (although the site reminds users of this option in some circumstances), and opinions about when to do this vary greatly between Stack Exchange users.