Click on the announcement you are interested in. Check the date of the publication (some opportunities in Europe might be already expired). If the announcement is still valid, look for the ‘contact details‘ section. There you can find free ways to get in touch with the hosts (website, e-mail, phone numbers, application form). Send the host/organization a message and wait for a reply.
Most of these opportunities and programs are free (you receive free food and accommodation in exchange for voluntary work). In some cases, you receive pocket money, please see: