As a rule of thumb, stay away from the border with the US (Tijuana, Juarez, El Paso, Monterrey, Laredo and so...) The rest of the country is a peaceful as it can be. If you remove those cities, the crime rates are way below the Canadian and American averages.

If you are not fluent in Spanish and haven't done a lot of traveling, I would stay on the touristy areas such as Cabo, Cancun, Puerto Vallarta etc....

It's not all dangerous. You can go around Mexico without ever getting intro trouble. People on the street don't really live as if the country was at war. Even in the hot spots like the north border or the states where there's a bigger prescence of the drug cartels life continues.