The post in Guatemala is slow. If you want to make sure that your mail arrives, and within a reasonable time, give your letters and cards directly to the post office (correo). The mail usually arrives within a week to 10 days at the destination.
Internet cafes are popping up more and more and they are easily found in Antigua. A Hotmail or Yahoo address can be the easiest and cheapest way to make contact with home.
In many places you can use your mobile phone. Check with your provider to see if you can use it in Guatemala. You can also buy phone cards. This is easy and fast, but expensive. Sometimes you can make a collect call request, to save you money. The international country code for Guatemala is 00502.