I have 20 or so bicycles and ride numerous types of bicycles. First you need to consider what type of biking you’d like to do. Where you’re most likely to bike. The type of roads or paths or trails you’d like to go on. Then you have to do a self evaluation of what your body is capable of as far as flexibility. I wouldn’t recommend a fixie for someone who isn’t an experienced cyclist who knows exactly what they want. I also wouldn’t recommend a single speed bicycle (like a cruiser) unless you know you’ll exclusively be doing flat terrain at a very casual pace. Flexibility determines what seating position is likely to be comfortable. More often than not a new cyclist who wants to do casual riding with possibly the occasional rough surface riding will want a hybrid bicycle. It’s a bicycle that’s fairly upright with tires that are narrow enough to be relatively fast but not so thin they aren’t comfortable on rougher pathways and roads. A regular Mtn bike is also a very versatile bicycle that may be a little slower on roads but more versatile and often more comfortable. When buying a Mtn bike be careful of going with an overly expensive one that may have race geometry that isn’t as comfortable because it’s very aggressive bent over position. A Mtn bike with only front suspension (known as a hard tail) should be more than sufficient. The Mtn bikes have mostly straight handlebars whereas the hybrids have a similar upright position but a bit narrower handlebar with a bit more curve or sweep in the bar. I’d recommend getting a bicycle with at least 3 speeds. Typically more expensive bicycles have more gears but that’s not always the case. Seats matters and often An experienced cyclist will change the seat on their bicycle to match their unique body needs. No one seat is best for everyone.
Test ride several bicycles at least before buying one. Every decent bicycle shop allows test rides and will give you a loaner helmet. A reputable bicycle shop will also help you find a bicycle that fits. In the current environment with so many bicycle shops sold out of bikes or having far fewer bicycles than normal there may be more pressure to sell you a bike that isn’t your size. Test riding a few bikes will help you get a better idea of what feels natural to you. When test riding the bike, try shifting through the gears while pedaling to see if it feels natural to you. I’ll leave it at that and let others add to what I’ve said. Bicycles are a great way to explore, get fresh air and stay fit and healthy. Good luck
I just saw you are living in Tokyo. It’s unlikely you’ll find many Mtn bikes there but then I’m no expert on Tokyo.