What's best to buy a Nikon or a Canon dslr?

I'm looking at the DSLR. Should I buy the D800 or other else?

If you're asking this question, you aren't ready for a DSLR.

There are other manufacturers, you don't need full-frame, and it takes around 18 months to learn to use any of them properly.

Start by finding out what a DSLR actually is and work from there.

The Nikon D800 is a high end pro camera, if you haven't used a dslr before you are spending lots of money on something you will struggle to use.
If you want to start using a dslr you are better off looking at the Nikon D3100 or the Canon EOS 1100D.
Both these entry level cameras can be used in automatic mode like a point and shoot while you are learning how to use all the options a dslr can give you.
There's no best between Nikon and Canon they both make excellent cameras and accessories.
My advice to you is to decide on the type of photography you want to do, then go to a large independent camera shop where you can get good advice. Spend a lot of time trying a number of cameras and asking questions of the staff before making your choice.

There are other makers other than Nikon / Canon, but they hold about 80% control of the market.
If I were you I'd look at a Nikon D3100 or Canon Rebel T3, but nice entry level cameras. Both around $475…