First: find the tooth and handle it correctly
Pick the tooth up by the crown, the white chewing surface on top, and never touch the root portion at all, which is exactly where the delicate cells needed for successful reattachment actually live and can easily be damaged. If it's visibly dirty, rinse it gently with plain water for no more than a few seconds — do not scrub it with anything, use any soap on it, or wrap it in a towel to try drying it off completely, since all of that damages the surface further and lowers the odds of success. Try to stay calm, since a clear head genuinely helps you handle the next few steps correctly, and it helps the injured person stay calm too, especially if it's a child who's already scared and looking to you for reassurance.