A standard tourist visa will suffice in most cases. These are issued for 30/90 days. Most Europeans will not need a visa, US citizens do, and a complete list of the visa type issued for each nationality is to be found at this page: Visa requirements page.
NOTE: This list may be out of date for some. If unsure or you think the situation for your nationality may have changed, contact the Bolivian Embassy for your country to seek confirmation. In particular, US citizens now have to pay a visa fee. See the US consular affairs page for information if you are a US citizen.
If you DO NOT require a visa, you need take no further action, you will be automatically issued with a tourist entry stamp for 30/90 days on your entry into the country.
If you DO require a visa you need to contact a Bolivian Embassy and arrange a visit to obtain one. This can be done at any Bolivian embassy around the world (it does not have to be the one for your country.) Check the table in the Visa requirements page for an approximation of visa fees.
If you want to stay for longer we will make some enquiries for you.