Before you book your flights, check your visa requirements. Not only for the country/countries that you are travelling to, but also the countries that you are transiting through. For some countries you will need to apply for a visa just to transit there (even if it’s just for an hour or two)!
Why before? Because you might be booking to travel for 3 months and 3 days and then you find out that you can travel to that destination for only 3 months (USA for example is very strict about this). Now you are paying to change your ticket or worse, buying a new one. Also, you might find out that you need to apply for a visa and that might take a while. Therefore, you may not be able to travel on your desired dates.
Some countries (like USA & Canada) won’t allow you to enter if you have a criminal record or have been charged before. To be able to travel to countries with these sorts of rules you will need to get special visas and these can take months.