The help page What should I do when someone answers my question? says:
The first thing you should do when someone answers your question is vote on it, like any other user does.
However, new users (which is probably the primary target group for this help page) may not have the Vote Up privilege yet (as it requires 15 reputation).
I guess they’ll see some kind of error message when trying to upvote before they have enough rep, but I think it should also be mentioned on this help page.