Flats are great if you have knee issues or just want normal shoes at the end of a ride. I tend to use SPD clipless for anything that isn't a ride into town due to habit, but I'm starting to use flats more when I commute so I can wear normal sneakers when I get to my destination.
If you can have a local and handy friend who cycles help, picking something up from facebook marketplace or Craigslist is the best way to get started. As long as the bike is mechanically sound and you don't grossly overpay, you can usually resell a bike with little or no loss in a year or two should you decide it wasn't the best fit/option.
If that isn't possible, buying from a shop is also fine - it will just cost more and you will lose a bit more money if you decide to offload the bike. The benefit would be having a shop for questions, and many will include free tune ups for a period of time after purchase which can be worth it.
You mentioned just getting into cycling but haven't said much about terrain. If you expect to stay on the road or paths, a flat-bar hybrid is probably the most versatile first bike. A mountain bike (accepts chunkier tires, frame is more slack, usually geared lower) would also be ok, but maybe not ideal unless you expect to be off-road at some point. I would probably steer you away from a road race setup (drop/curly bars) if you are just getting started as they tend to be less versatile unless you get something beefy and gravel oriented. Once you figure out where you will cycle and what style bike you want then spend the money to get a more targeted setup (ie road race, touring, gravel, mountain, etc).
Stick with the big names if you buy used - Trek, Giant/Liv, Specialized, Cannondale, etc (not an exhaustive list) instead of Walmart or box store brands. The big brands are more likely to be repairable and sometimes shops won't work on box store bikes.
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