What are some social customs and norms I should know before going to Guatemala?
One thing you may notice is that as you make friends with people, your personal space bubble tends to be a lot smaller, i.e. its not uncommon for friends talking to you to be closer than you are used to.
You may see women breastfeeding openly, this is normal.
As a foreigner, people may attempt to overcharge you, feel free to negotiate on prices that seem too high. Also, consider asking locals what an acceptable price is for whatever you are purchasing, this can help in your negotiating.
It is common for people to refer to each other by their defining features, i.e. if you have darker skin you may be called "negrito", or for lighter skin "ganche" or "gringo". It's not uncommon for people to call larger people "gordito" or "flaquito" for those that are thin. Don't take offense to this, it is simply part of the culture and is not something intended to be offensive.
The above advice is based on my experience living in Guatemala for a few years, I did live further south, along the western coast of Guatemala and near Quetzaltenango, but when I visited Peten, the culture seemed fairly similar to what I experienced in other parts of Guatemala.
One last thing, don't assume because you are used to something being a certain way it is better. Don't say anything like "well that's the way we do it in my country". This should be said when visiting anywhere away from your home country.