Upvote is when you click on the ^ beside the $ sign, the circle will go solid blue when done. This is your way of saying you "like" the post, you can also be rewarded if done early.

Resteem is sharing someones post on your own feed, click on the arrow beside the "reply" to resteem.

Flagging a post or comment should be avoided unless the post is bad or the comment is bad, doing so removes rewards for that post and limits the post.