IME knowing when and where to go is very important (it sounds obvious but you'll - presumably - want to avoid peak times such as school holidays, religious festivals etc).
Deciding how to get there: plane, train, bus or car.
Time of year is important, too: sun, rain, the right season etc.
And of course accommodation: basic, budget, comfortable, luxury.
And then duration: a week, fortnight, month, season.
Also what the are attractions there: culture, history, acivities, sports, beaches, shopping, day-trips etc.
Once you've have the answers then you'll be able to narrow down your choices.
enjoy.