A few times a year I get this question posed to me. What should I buy?
What are you using the camera for, do you plan to make money with it? Are you willing to learn how to shoot manual? Do you need something thats water resistant. A million other questions you can ask yourself as well.
If you plan on making money with the camera (weddings, portraits, ext) then you have to consider quality of camera and durability and a number of other factors. Should you choose Canon or Nikon, it doesn't matter. You do what to choose Canon or Nikon though because we know that they will be around in 5-10-20 years, also lots and lots of accessories. You're going to spend between $1000-$3000 on a good DSLR. Anything under that price range you start to risk image quality and sensitivity of the sensor. There is a million youtube videos out there on choosing your first DSLR, i'm not going to copy that info or post any different. I agree with most of the info out there on choosing your first dslr.
If you just want a camera to take photos of your cat, dog, kid; you are limitless in your choices. Depending on the quality you can pay anywhere from $40-$600 for a basic camera or DSLR. Nikon & Canon offer very affordable entry level DSLRs that will get you started on the right foot.
Before you buy anything though you should do lots of research. SnapSort.com offers a great comparison between different cameras and usually will tell you which is better at the beginning of the page.