If you're looking at someone else's question, you're limited to just clicking the blue arrows in the vote box next to the answer(s) you think are helpful or unhelpful (and the question itself, if that was helpful or not).
If it's your own question, like this one, then in addition to upvoting a good answer, you as the asker can also select the best answer. There is a large check mark under each vote box for each answer to a question; whichever question you find the most helpful, select as the best.
edit: yes, as pointed out below, you can't vote at all until you have 100 PGC points. After that, you can do all these things.