1. Read! Read widely - different genres, different settings, different nationalities and cultures. Don't read only modern fantasy - read historical fiction (and not just set in the west), read important books by authors from the non-English-speaking world, etc. Read the classics. Read challenging books - those that require focus and mental exertion to understand, and those that push you out of your comfort zone.
2. Read! Read craft books. I don't think you necessarily need to do a lot of writing exercises (I certainly haven't), but I do think you can absorb a lot by simply learning to think about writing as something that can be learned and practiced, as a skill that can be developed. Look at your favorite pieces of writing and try to understand why they affect you.
3. Experience things. Go live a good, rich, interesting life. Do things that are hard and uncomfortable and let yourself feel strong feelings about it. Go rock-climbing. Take a dance class. Take a karate class. Go to a new city by yourself. Go somewhere you can't read the street signs. Your characters may need to be in a situation where they feel frightened and unsure of themselves... when you describe that feeling, it helps to draw on authentic experiences.
4. Study people. Study different cultures. Look at how people interact. Look at how people show (or hide, or attempt to hide) their emotions. Look at how they talk to people they like vs. people they aren't comfortable with. Listen to the words they use, the slang, the ways they show respect. Look at how personalities and experiences and culture and environment can shape people and their actions.
5. Oh, and write. Try your best, but don't expect it to be perfect. Humans aren't perfect, and you won't be able to create a perfect piece of writing. Give yourself grace to learn as you go. Just because you write it doesn't mean it needs to be published or shared. It's ok to write things just for practice and just for yourself.
Check out C. J. Brightley latest release The Wraith and The Rose, a fantasy reimagining of the Scarlet Pimpernel with fairies and magic.