What is the best way to break the writing barrier and get the words to flow?

I’ve never been a writer and I can’t remember the last time I even sat down and wrote a story. I used to write short things when I was a kid but really not since I got out of high school. I’ve never really had a story or character ideas, but I’ve been reading a lot of old D&D stuff and it inspired me to give it a try and write a fantasy book. It’s been a great exercise and I’ve really enjoyed brainstorming and coming up with stuff I like and setting up a world, but I just can’t seem to get it down on paper. I can’t seem to get the words to flow and it’s really slowing me down and making it not really fun. Any suggestions?