The /s/ sound in both Mexico and Spain are relatively similar. In Spain, the point of contact for the tongue is slightly further back in the mouth, making a slithgly more raspy sound- not quite an English /ʃ/ (or "sh" sound) because the mouth is not rounded, but similar tongue placement. The /s/ in Mexico and Latin America is said with the tongue in a similar position to how the /s/ is pronounced in most English words. Again, this difference is VERY subtle and almost not worth discussing unless you are a professional linguist.
Mexican and general Latin American Spanish also do not have the /θ/ sound (basically the English "th" sound). So, in Latin America the "z", "ce" or "ci" will be pronounced like you would an "s", whereas most people from Spain would pronounce those sounds with that /θ/ (or english "th") sound.
Examples: zapato, ciencia, cero, gracias
Whether in Spain or Latin America, the correct pronunciation of the letter "s" is similar to English, with the noted slight variation mentioned above. So "estoy", "sesenta" and "sello" would all sound pretty much the same if said by a Spaniard or someone from Mexico.
*Note: I'm speaking of generally accepted Spanish and Latin American pronunciation. As with Standard American English, there will always be variations within the particular regions of a country. Ever had a conversation with someone from Mississippi and Boston at the same time? This can me highly entertaining if you enjoy phonetics and language :)