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What's the right way to load kernel file through boot loader in a memory (the kernel is written with C, for example, "Hello world."). Um, I don't need someone to tell me, exactly how to write in boot loader the "Hello world", but how to load another file (that is a kernel file)?

It depends on what bootloader you are using. If you have GRUB set up properly on a floppy or floppy image, with the kernel in the root directory, you can put the lines:

title Hello World
kernel (fd0)/kernel

in grub/menu.lst

Setting up a floppy from scratch with a bootloader is a bit more tricky, and depends a lot on your OS. If you run Linux, look at JamesM's kernel tutorial (http://www.jamesmolloy.co.uk/tutorial_html/index.html), and if you run Windows, look at bkerndev's tutorial (http://www.osdever.net/bkerndev/index.php). Both have good basic bootloader/image/emulator setups, and are actually almost identical in that respect.

Another way is that you have your own bootloader...
It's like redundancy work, but it's good experience for you.
If you decide to make your own bootloader, Wiki's babystep1~babystep8 is good reference.
http://wiki.osdev.org/Babystep1