Depending on your nationality, you might need a visa for some international crossings, so it's important to check what you'll need before you travel. The best place to check is the destination country's government website. Googling 'Do I need a visa to travel to the country you're travelling to?' will give you links to the best resources for the information you need.
Getting a visa can take several weeks, so it's best to apply well before your travel date.
For information about travelling between the UK and the EU, click here.