usually in their senior year they go into the nfl draft, but they can leave earlier, its upto them when they want to put themselves into the draft which year, some have left early etc.
Basically the draft is the worst team in the nfl picks first, then it works its way back to the team that played best in the league, then worst team picks again and so on.
Teams can trade players and other pick positions for like one higher in the draft.
Usually the players picked in the first round of picks by the teams in the draft, are considered the best players coming out of college, but thats not always true, tom brady one of the best players in the league was picked in the 6th round if my memory serves me right.