When you enter Guatemala – by land, air, sea or river – you should simply have to fill out straightforward immigration and customs forms. In the normal course of things you should not have to pay a cent.
However, immigration officials sometimes request unofficial fees from travelers. To determine whether these are legitimate, you can ask for un recibo (a receipt). You may find that the fee is dropped. When in doubt, try to observe what, if anything, other travelers are paying before it's your turn (Q10 is the standard, nonstandard fee).
To enter Guatemala, you need a valid passport.
Normally customs officers won't look seriously in your luggage and may not look at all. Guatemala restricts import/export of pretty much the same things as everybody else (weapons, drugs, large amounts of cash, etc).
Many nationalities do not require tourist visas and will be given a 90-day stay upon entry, though citizens of some countries do need visas.
Citizens of the US, Canada, EU countries, Norway, Switzerland, Australia, New Zealand, Israel and Japan are among those who do not need a visa for tourist visits to Guatemala. On entry into Guatemala you will normally be given a 90-day stay. (The number '90' will be written in the stamp in your passport.)
Citizens of some Eastern European countries are among those who do need visas to visit Guatemala. Inquire at a Guatemalan embassy well in advance of travel.
In August of 2006 Guatemala joined the Centro America 4 (CA-4) trading agreement with Nicaragua, Honduras and El Salvador. Designed to facilitate the movement of people and goods around the region, it has one major effect on foreign visitors – upon entry to the CA-4 region, travelers are given a 90-day stay for the entire region. You can get this extended once, for an additional 90 days, for around Q120. The exact requirements change with each government, but here's how it was working in Guatemala at the time of writing: you needed to go to the Departamento de Extranjería, with all of the following:
Two black-and-white cédula-sized photos on matte paperA valid passportTwo copies of the photo page of your passport and one copy of the page with the entry stamp on itA credit (not debit) card with a photocopy of both of its sides (or US$400 worth of travelers' checks or a ticket out of the country or proof of flight reservation on travel agency letterhead, signed and sealed by a travel agent).
Extensions can take up to a week to process, but this period is also very flexible – it's worth asking about before you start the process.
If you have been in the CA-4 for your original 90 days and a 90-day extension, you must leave the region for 72 hours (Belize and Mexico are the most obvious, easiest options), after which you can return to the region to start all over again. Some foreigners have been repeating this cycle for years.
Visa regulations are subject to change – it's always worth checking with a Guatemalan embassy before you go.