You will need to apply for a visa or other entry authorisation in some countries. Apply for a visa well in advance of your departure.
You also need to apply for an entry permit in the UK, which has introduced the Electronic Travel Authorisation (ETA) requirement. Find out how to apply for ETA on gov.uk.
How to get a visa
You can purchase a visa by contacting the embassy or consulate of the country in question directly or through the travel agency CWT.
Log in to CWT here and then click on Visas under Useful links.
You can register as a user in CWT by contacting the SDU Travel Office. Read more about CWT here.
Apply for a visa well in advance of your departure.