Welcome to the board. A lot of design choice depends on how you are quilting. On a DSM? On a longarm? If so, do you use pantographs, robotics, or free motion? Are you comfortable with rulers? While you can technically quilt any design using any machine, the actual choice often depends on your equipment and your skill level.
I get a lot of design inspiration from other quilters on youtube, in blogs, or in books. Angela Walters is a good one to watch but there are lots of others. It also helps to take classes at quilt shows. Some of these focus specifically on how to decide on what to quilt where. But for most of us it's an ongoing question. I have a quilt loaded on the longarm right now. I've stabilized it with SITD, but I'm still waiting for inspiration for what designs to use with the actual quilting. Good luck!