Definitely don't worry about crashing. Mountain bikes are made to take abuse, & if anything does break it will likely be components (& not the frame), when can be replaced as they break relatively cheaply.

As far as which bike you get, I'd go to a local bike shop & at least test ride a few to get an idea of what size/style bike would best suit you. If you find one there you like, go ahead & buy it. Buying from the shops has many perks if it's a good bike shop--setting it up for your height (& weight if it has an air-adjustable fork) is common courtesy, & you may even get the first tune up free or all free for the first year, etc.

If you don't find a bike you really like at the shop, then at least you've gained some knowledge about what you did & didn't like about the bikes you tried out & you can apply that to whatever else you may be looking at. And for gear, you'll need a helmet. You'll likely want gloves & a pair of riding shorts with a chamois, & possible a moisture-wicking jersey for hot days. Helmets range anywhere from $20 at box stores to several hundred for really nice carbon fiber helmets. They're all roughly the same as far as protection, it's really a matter of comfort, weight, & aerodynamics for the more expensive ones. Gloves will be ~$20, jerseys start at ~$50 in my shop but can be had cheaper online, & shorts are pretty dang expensive--I never pay attention to the lycra ones because I don't like them, but the baggy style biking shorts start at ~$80 at my shop.

The only note I have to add is that I'm not encouraging you to go into a bike shop & test out all their bikes with the sole intention of buying cheaper online. If you test them & don't like them, that's one thing, but finding one you really like there & then buying the same model cheaper online is generally frowned upon around here.