Zona Norte is the notorious red light district in the north part of Tijuana, not far from the U.S. border. The primary block is bounded by Calle Coahuila and Callejon Coahuila (known as "the alley") between the avenues Constitucion and Ninos Heroes.
The neighborhood has a mix of "gringo bars" and "local bars," but non-locals are quite welcome in either ... tourism money is what makes the economic wheels turn. The primary gringo bars are Adelita Bar and the Chicago Club. Dollars are welcome.
The primary reason anyone goes to this neighborhood is for the girls, who are plentiful both in the bars and on the sidewalks, especially the sidewalks of "the alley." Prices typically start at $20 (street girls) and $50 (bar girls), plus 5 to 10 for half an hour's rooom rent in one of the hotels above the bars. Prostitution is legal in the district and there is a police station on Constitucion. The neighborhood is reasonably safe if you're polite, careful, observant, and stay out of dark alleys.
Traveling from the United States, the usual advice is to park at one of the lots on the U.S. side of the border, walk across, and get a $5 taxi ride to Adelita's (pronounced ah-day-leet-ahs).
The "official" Tijuana FAQ is at the World Sex Guide site.