The whole family is off to Guatemala tomorrow morning for a week of adventure, courtesy of my mom... this is the first time I've traveled internationally, so it's a whole new experience.
Have fun and a safe voyage! I think being your first time out of the US, you'll find Guatemala exceedingly exotic. And even more exotic than traveling to say Argentina or Mexico, because the size and development of Guatemala is very different, economically speaking.
As a first timer, just be vigilant in the streets. If Guatemala is anything like Mexico, you will be surprised to find a number of poor children and older people begging in the streets all the time (and I mean all the time) as the government struggles to enforce universal education, and provide for the poor. You will find ambulant vendors trying to sell tourists something incessantly. This is something that can be overwhelming for most Americans when they go, say to beach resorts or cities. Foreigners are seen as sources of money, basically, and that is a nation where on average income is low so the market forces as strong.
Otherwise Guatemala is like the Ireland of Central America; everything is so green! I imagine you've been told about the topography! Are you visiting the Mayan centers? My Mayan experience was on the other side of the border, in the Mexican states of Chiapas, Yucatan and Quintana Roo. Bring back some handcrafted Mayan style souvenirs. People there are fiercely proud of their history, and you'll find the artisan crafts are a major industry in those countries.
For me, when I was about 9 or 10 years old, it was a phenomenal adventure. We drove a 19ft RV, from our house in Mexico City, and and drove around the Yucatan state peninsula, east to the state of Quintana Roo, stopping at Can Cun and then turning southeast back to Chiapas state. We drove past the border with Guatemala.