Agree with prior poster. First, I think about places I want to go. Then, I think about places that might be combined with a train or plane to the first place. I think about which destinations require a week and for which destinations a week would be too long. I look up safety, State Dept., warnings. I think about weather/time of year.
The RS guidebooks are the best ones out there. Definitely buy/borrow an up-to-date one.
But publication lags mean even the newest ones are out of date. So it is necessary to supplement.
I check online about visa requirements well in advance. I check my passport will be valid for six months after my return date…also well in advance.
I do watch short travel videos. I agree that Somebody Feed Phil offers great food tips. I check hotel reviews on TripAdvisor and hotels’ numeric scores on booking.com. I relish friends’ suggestions. Websites are mandatory for museums.
Walking tours are another resource. Early on in a trip, tips-only walk offers a good overview. Specialty walks with price tags are also good: Context, Culinary Backstreets, etc. Free walks with greeters are wonderful: check online for Greeters four weeks’ ahead.
For flights, I check online for which airlines fly to a destination. I aim for nonstop when possible.
And I pack lightly! Carry-on is the way to go even if you check them.
Now for a first trip in a long time or a trip that is complex due to language or anything else, you might consider a group tour. Most include free time, too, or you can add some time at the end. Someone else worries about most details. You just enjoy and learn more about how you’d like to travel.