Just go. Now. Don't think about it. You can do your thinking about what you're going to do while you're doing something. That's the first step. Once you're done doing something, ANYTHING, maybe check out blogs and other annoying fit gurus like on Facebook or Instagram for inspiration or ideas. But don't do that first, that is counterproductive. Do something first. The more you do something the more it becomes routine. And the more you do something the more you'll see results and once you start seeing results the more you want to maintain those results. But don't be fooled into minor results because those can give you false confidence. Just keep doing that something you used to do. I started out biking and though I've moved onto weights and kettlebells and SUP/kayak and hiking and cross country skiing I still always fall back on biking. "What should I do?" If I can't think of something in 5 minutes, "might as well bike."
Basically create motivation and eventually the motivation manifests itself into your life.