Teotihuacan is 50 km north-east of Mexico City, about an hour by car if traffic is reasonable. There are five gates around the site; the main visitor entrance is Gate 1, by the Ciudadela, where the buses arrive.
By bus from Mexico City the cheapest and easiest option is the Autobuses Teotihuacán service from the Terminal del Norte (TAPO is also possible but less frequent). Buses leave every fifteen minutes from around 6 am to 5 pm and take about an hour. Coming back, buses run from Gate 2 (by the museum) and Gate 1.
By private taxi or ride-hail from central Mexico City, expect around 600–800 pesos one-way; many drivers will agree a round-trip rate including waiting. Several reputable tour operators in Mexico City run guided day trips with a stop at the Basilica of Guadalupe en route.
Plan to be at the gate before opening (9 am) if you want to beat the worst of the heat and the crowds, especially in March–May and on Sunday mornings when Mexican nationals get in free. Bring water, sun protection, and good walking shoes — the site is enormous.