Don't be dumb -- choose the path of least resistance
Whatever you do, don't be like Drake. Going from zero to 100, real quick, will do nothing but leave you feeling defeated. Start out with a couple 20-minute sessions per week. That way, if you achieve just that, you're golden; if you do more, you'll feel like you just completed a marathon (in a good way).
When it comes to choosing an activity, think about what you're familiar with. Did you bike a lot as a kid, or run track in high school? "Choose the path of least resistance, what feels easiest," Cook Harris advises. If you have even the most basic understanding of a form of exercise, or at least remember that you enjoyed at some point, you're bound to feel more confident.