There is an up & down triangle before the question and answer, just hit them and you can vote for it. Up Triangle refers to upvote and down triangle refers to downvote/negative voting.
For Questions and answers, you can do both upvote & downvote, while for comments you can only do upvote, you can't downvote a comment.
But for voting you require at least 15 reputations, see this privileges page.
Whenever you upvote an answer or a question, the user will get
+10 reputation (you can not upvote your own question/answer or comments) while upvoting a comment doesn't reward in reputation.
For downvote, if you downvote a question it doesn't costs you, while if you downvote any answer, you will lose
-1 from your reputation too. If someone else downvotes your post (question or answer) then you lose
-2 from your reputation.