To enter Guatemala, you need a valid passport.
On entry 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 at land border 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).
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.
Guatemala is part of the Centro America 4 (CA-4) trading agreement with Nicaragua, Honduras and El Salvador. 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. Do this at the Departamento de Extranjería in Guatemala City. Along with the fee you currently need two passport photos, photocopies of of your passport and the front/back of a valid credit card. Extensions are usually issued in 24 hours.
Travelers may find it easier to leave the CA-4 region (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.