Check the expiration date of your passport and credit cards. Extend if needed.
There exist a few vaccinations which are really recommended in some countries. Call the tropical medicine for further information. It is expensive but normally it is just once in your life.
Save in your email or dropbox: Transportation tickets (if not on an app), Hostel reservations, Insurance information, Contact info for credit card companies, Emergency contacts and important addresses and a list with passwords (coded) plus a scan of your important documents (passport, personal ID, driver’s license and international vaccination card)
Check if you need to deregister from your home country. I have to when travelling longer than three months.
Buy your first transportation. Check if you need an onward ticket to enter (e.g. US). If you do not know how long you will stay in a country buy an onward ticket and cancel it. Most airlines have a 24-hour cancelation policy so you can purchase a ticket, print it, and then cancel it. Another option is to purchase a cheap bus ticket to another country.
All the information you need while traveling including visa requirements, currency, electricity, communication info and more: https://www.thebasetrip.com
Check the weather. Adapt your clothes.
Do a small research for each country/place what you would like to visit.
Organize your first 2-3 nights. It is nice to stay spontaneous and go with the flow.
Learn a few words (e.g. thank you, sorry) in the language of your next destination. A way to conjure a smile on somebody’s face.
Learn a new language: duolingo.com (free. exist as app as well). It is fun and addictive. You learn the orthography, auding and translating (no speech practices) and the possibility to repeat the learned words.