A picture is worth of thousands of words. Look at this big sign at the Gate 5 of Terminal 3 of Beijing Capital International Airport.
You can take bus directly from Arrival gate of Beijing airport to Tianjin.
The bus starts from 9:00 AM to 22:00, and interval is 30 minutes. Ticket price is 70 RMB (or 10 USD) per person.
You can also take the bus at Gate 11 of Terminal 2 of Beijing Airport
The departure and arrival station in Tianjin is at the corner of Nanjing Road, and Shanxi Road