Here’s my advice:
Choose two languages that you have not yet mastered and which seem like they would be worth learning.
Commit to studying them consistently, daily if possible, for as long as necessary to become all but fluent in them. This might take years.
The process of choosing, the first step, should be challenging and exciting all by itself. Available are all the languages that have ever been written down in all of history. We could study ancient Persian, Sanskrit, Latin, classical Chinese, or we could just as easily learn Farsi or Hindi or Italian or Mandarin. With such a buffet, choosing the top two is a big task, which can hardly fail to make the soul giddy with possibility.
The two languages chosen, it’s time to get started on studying one of them. It doesn’t much matter how. I recommend Duolingo, but there are countless options. The point is to get started. If the first choice isn’t working, it’s always possible to jump to a different approach. Restart as often as necessary, with as many different methods as needed; getting started, and not stopping, is the most important thing.
But why two languages, then?
Whenever one language gets too boring or challenging or uninspiring, it’s always possible to jump to the other one for a time.
And if learning one language is ever too easy, it’s easy and exciting to add another and be working on both simultaneously.
Get started, and stick with it. The rest will take care of itself.