This question is asked at least once per week by someone looking to change their life through starting CrossFit. Which, by the way, is a great idea. The confidence you gain through fitness is outstanding. You believe in your ability to do hard things, and you learn what it takes to find success. These skills naturally spill over into your career and personal life, as well.
The problem with online programs is that they don't have the ability to actually coach you through the movements. They will show video demos, and some coaches may even do a video review of your movements. None of that matters if you don't remember to keep your back flat on a deadlift and end up with a herniated disc (at worst) or a back which makes it painful to walk (at best). You need a coach to properly start CrossFit.
Once you understand how to move through the workouts safely, you can evaluate what program you should follow. Whether this is CrossFit's main site workout of the day, another online program, or a local CrossFit box (like us!) doesn't matter AS MUCH AS you following through with it. Consistency over everything.
Technically, you don't NEED this. But let's be honest, how much easier is it to go to the gym when you have someone coming with you? Studies have even shown you feel like your workout is easier when you're in a group. So, if you're following an online program, get some friends together and follow it together. If you're at a gym, you have that taken care of already.
No, you don't need to buy a weight belt, Olympic lifting shoes, and handguards in order to start CrossFit. Chances are you won't be using heavy enough weights to need a belt, you'll be snatching a PVC pipe or a training bar so you don't need lifting shoes, and you won't be doing kipping or butterfly pull-ups, so no grips needed. You need basic equipment provided by your gym (or if you're in your garage check out these packages from Rogue) and shorts, t-shirts, socks, and a firm shoe. Stay away from running shoes which don't function very well in a CrossFit environment because they're bad for lifting weighted objects.
We put brand new CrossFitters through our OnRamp program, a three-class introduction which gets you up to speed on what CrossFit is, what movements you need to know, and how to be successful and safe in our program. If you're brand new to CrossFit and your gym doesn't have anything like this, back away slowly. Then run. Preferably to our doors.
CrossFit is going to challenge you. It will be the hardest workout program you've ever done. The combination of cardio, weightlifting, and gymnastics forces your body to work extremely hard - and therefore to find great results. If you walk in convinced it is "too hard" for you, or that you "will never be able to do it," then that is what you will search for. You'll find verification of those statements being true. But, if you develop the right mindset, you'll find the results you've always truly been looking for.
Last but certainly not least, let's address the idea that you need to be in shape to start CrossFit. Nothing could be farther from the truth. Take running as an example. Some studies indicate as many as 75% of all long-distance runners are injured each year, yet you never hear someone say they have to get into shape to start running. In fact, it's normally the first choice! Go to the Globogym and slog away on the treadmill for an hour. Rinse wash repeat. Get bored and frustrated and quit.
Do you want to be ready for CrossFit? Start.
You'll get in shape for CrossFit by doing CrossFit.