Here's the deal when crossing the border into Guatemala from Belize (I crossed at Benque Viejo de Carmen, near San Ignacio).
First, take a taxi (no cheaper option exists, except walking - it’s a pretty short distance), from Benque to the border. When you get out, there are loads of money-changers - I would suggest changing about $20 into the Guatemalan quintiles.
Once through customs (after paying the Belize departure tax), you'll be inundated with men trying to sell you a ride to just about anywhere. They work in cooperatives and generally one person will represent the entire lot. Generally there are three ways to travel in Guatemala:
1) Taxi - not recommended. These guys (they are all men) charge A LOT because of the conditions of the roads there. They have to replace their cars often (sometimes once a year!).
2) Minibuses - These look like the old Volkswagon "peace" buses. The advantage is that you can get with several others going to the same location, which makes you feel a bit more comfortable. The disadvantage is that you will pay more than you should. When we traveled to Tikal from the border in one of these, we paid about $7 (it’s a 1 hour and 45 minute drive). We later found out that we should have paid about $3. Just make sure you haggle hard with the driver before arranging a price, and watch out that they don't change the price on you. The only other thing to keep in mind (but don't get paranoid over) is that these have the word tourismo written in huge lettering across the front and are a "riding" target for the thieves that like to hang out between the border and El Remate (on the way to Tikal).
3) Collectivos are the public transportation. Don't look around for a large public bus system because it doesn't exist. These buses aren't much bigger than the minibuses, but you'll know the difference because only locals tend to use these. These are by far the cheapest way to go. In most cases, the driver will not speak English, but if you can say El Remate (a major stop on the way to Tikal), you're in good shape. The other thing you should know... drivers are in it to make money, so plan to be cramped in the bus with loads of other people. These are small buses, but we still had 20 people (including two small children) in our bus on the ride from El Remate back to the border!