Caveat: New cameras are introduced all the time. Today’s recommendations may not be good in the future.
When you buy a serious camera, you’re actually buying into a system. This means the brand of camera you purchase is important. Once you purchase the camera, you will also be looking at lenses, flashes, and other accessories. Tripods and some other gear work with almost all cameras, but the lenses and flash units, which are the most expensive part of your accessories, are camera specific.
My recommendation is to go with either Canon or Nikon for a DSLR system. Go with Sony for a mirrorless system. These are the largest manufacturers, and are the most likely to be in business long term. Also, if a product is made, it’s guaranteed it will be made for these two camera types.
There are three levels of serious camera: entry, enthusiast and professional. Entry level units have quite a bit of capability. Many professionals work with cameras that are listed as enthusiast level. Professional cameras have more weather sealing, usually take more frames per second, and are more rugged. The price range for entry level cameras is $400~$600. Enthusiast cameras run from $900 to $2500, and professional cameras run from $3000 to over $6000. Entry level cameras can often be purchased with lenses, while enthusiast and professional cameras are always sold as body only. You have to purchase the lenses separately.
The lenses that come with the entry level cameras, called “kit lenses” are okay lenses, but are not terrific. Ideally, your camera will come with a wide angle to normal lens and a zoom telephoto.
The wide angle to normal lens is for inside and closer photography, such as family gatherings. The zoom telephoto is for things that are further away, such as wildlife and sporting events. Kit lenses are slow lenses, which means they’re not particularly good in low light. They’ll do a good job in good light.
In Nikon, the entry level cameras have a thousand number, such as D3400 or D5500. Each new version has a higher number than the previous version. Canon uses the term “Rebel” for their entry level cameras, such as “Rebel T6.” The higher the number, the more capable the camera. Sony is still a work in progress, but the “A” series is serious. The Ar” series is super serious.
Enthusiast level cameras vary quite a bit, although they often have larger sensors, work better in low light, and will take more photos per second.
If you’re considering a professional camera, you’re probably way beyond the information in this posting. Professional cameras often have fewer features than entry level and enthusiast level cameras. As an example, they seldom have a built in flash, as professionals always use a flash unit of some kind for artificial lighting.
Which camera you start out with depends upon your level of interest and your pocketbook. Remember that even a $1000 enthusiast camera will require purchase of a lens separately. Lenses run from $150 to over $10,000. Expect to spend $400+ for decent quality, reasonably fast (able to take photos in low light) lenses, and much more for top end lenses.
BHPhotovideo keeps a page on DSLRs here.