Tiwanaku is located only 20km from Lake Titicaca and 2 hours from La Paz. To get there you can catch a mini bus there from the Cemetery district (the corner of Aliaga and Eyzaguirre). They depart every 30 mins and take about and hour and a half. Once there you can hire a guide from outside the museum but if you want to guarantee a guide it is usually better to arrange an English speaking guide from La Paz. Expect to pay around U$10-12 for a one day tour including entrance into both museums and the site.
Tiwanaku (also Tiahuanaco) lies in the Bolivian Andes, 3,750m above sea-level. It is located some 24km from the shores of Lake Titicaca. Some have hypothesised that its modern name is a corruption of the Aymara term "taypikala", meaning "stone in the centre".