Getting to the Metro from Mexico City Airport is not as easy as you would think. If you are new to Mexico, get used to the fact that there are very often NO SIGNS to anything.
But it is only a 15 minute walk, out of the airport, along the airport path, then through a market, into the subway station. Here is a map showing you where you need to walk, save this to your phone. This gets you to Pantitlan Station, which includes lines 1, 5, A and 9.
And here is a satellite map of the same area.