If you are a standout playing college football, Pro teams have talent scouts that make contact with your college and express their interest in you. There are rules that govern whether they can have direct contact with you prior to you becoming "eligible". Once you are "eligible" you can be invited to a ----- tryout. At the tryout you will be put through Football Drills that include speed, agility, strength and knowledge of the game of football. You and other participants will be evaluated and considered for the upcoming Pro Football Draft. If you are a good prospect you will be selected in the Draft. If not, you can still be invited to a Pro Football Pre-Season camp as a "Free Agent". This means that you have enough talent that the team wants to evaluate you further.
I hope you find this helpful. Good Luck and Enjoy the great game of football.