It depends on what sport you are talking about to be very specific. But in most situations, such as football and basketball and baseball, the team that finished last in their league, gets to pick first. So since they were the worst team, they get the best college player as the first round draft pick. It’s also like monopoly; teams trade one draft pick or two draft picks to another team in order to move up a few spots and get a better player for themselves. The player who is picked last in the draft (well, he is really not picked per se, because he is the last one, and therefore whatever team is left with the last pick is stuck with him), gets a prize and a vacation and a lot of good stuff. Not because he is bad or else he would not be in the draft, but because he just finished last, and it’s a tradition.
P.S. You don’t have to use all of that in your answer of course. Some of it was just for information :-)