Some guidebooks are still wrong, they now charge 80 Bolivianos (July 2014) for Extranjeros (foreigners). Citizens - locals pay about 10 bob). Getting there is rather easy. If you're staying in El Centro (the city), take a taxi to, or a minibus with the "Cementerio" flap on the window to the Cemetery. Across the street from the cemetery's main gate, there are florists and to the right of this area are minibuses headed to Tiwanaku, which is usually a stop on route to the border at Desaguadero.
The trip should take about 6 hours in total, so it's ideal to take a bus to Tiwanaku around 8:00am, do a 1.5 - 2 hour tour, buy souvenirs and check out the museums, and head back to La Paz getting there around 2:00pm or 2:30pm.
You shouldn't pay more than 8-15 bob (15 Bolivianos as of July 2014) for the bus to the entrance to the modern village of Tiwanaku. It is 2-3km from the entrance of the village on the main road (90 mins by bus from La Paz, a bit more to the town itself) to the archecological sites, so try to find a direct bus whenever possible. Buy some snacks and drinks before going anywhere close to the archeological sites, because prices may double and triple as one approaches.
Also, when the drivers head up to El Alto on the road to Tiwanaku, they will most likely pick up more passengers if the bus or minibus isn't full. There usually isn't much traffic on the road out of El Alto, meaning that the bus may not become slower and/or more crowded thereafter. Some drivers drive fast so they won't waste time, but they are experienced so don't feel nervous. You might also want to choose to sit near a window or out of the sunlight. Also, note that the altitude at Tiwanaku is a bit higher than that of El Alto's, as a tourist you're most likely not going to get used to this. The 'sorochi' pill has been known to be effective against altitude sickness - but it is dangerous to take unknown pills but safe to follow the locals in chewing coca leaves.
You can book a tour when you arrive there for 95 Bolivianos (July 2014) per group (the bigger the group, the less you pay per person). The tours are very helpful and informative (otherwise you probably won't know what's going on) and last 1.5 - 2 hours. (Note: the tours are in Spanish).
You can also book a tour through the many tour operators in La Paz. Most of these appear to be in Spanish only.
This is where you should stay, nowhere else to go to. There are only small villages in the surrounding areas of Tiwanaku and the site around it.
The remains of a Pre-Incan civilization. This monumental city in the Bolivian highlands 13,000 feet above sea level and one of 754 recognized World Heritage Sites, Tiwanaku is surrounded by mountain ranges, with Lake Titicaca on its west side (though not visible). The massive, solid blocks of a stone not indigenous to the flat plateau give rise to the site's nickname, "the Stonehenge of the Americas"--and, over the years, they have given rise to some other worldly theories of how the site came to be. The museum contains most of the amazing things built by the Tiwanakan people, pictures aren't allowed but sometimes can be taken. Something interesting is in the museum: a skeleton was recovered that is about 13,000 years old. This place is quite magical.
Take as many pictures as you can. This site is beautiful, the architecture and style is impressive. Don't cross over signs or anything, you'll upset the security guards there and might be kicked off grounds. You get quite a lot of freedom there, to walk wherever you want. Visit Lake Titicaca at a 30min drive the boat ride is about $5. Visit the Pariti Island with more ruins (not fenced, Aymara tombs and a ceramic museam (this is an all day trip and can cost between $20 to $30. The Main Plaza, numerous sculptures around the plaza, a 400 year old colonial Church, and asked the neighbors to show their personal monolitos, mommies, ceramics, their families decades ago. The Fernandez family in the main plaza has a 9ft original monolito (monolito Zunagua)in thier back yard.
Many indigenous women will be selling pottery, scarves, clothes,crafts, and other handmade things. They do not really charge much, and these items are worth it for the price. You'll probably only come here once in your life, so you might want to make sure to buy things you'll have to remember. Some weigh a couple of pounds, so when packing your luggage at the end of visiting Bolivia, put these things in your carrying bag.
There are a couple of restaurants near the museum. They are a bit expensive but the food is exquisite. Make sure to be careful eating fish or other seafoods, they should be well cooked, especially since you're probably not a native.
You'll find people selling water and beverages around. At the restaurants, they have more variety.
You don't want to leave this area. The area surrounding Tiwanaku and the small town around it is quite barren. You'll see a small mountain range to the West of the ruins and other mountains all around. The main attraction is this site so don't wander around.