OSdev is a pretty good starting point for basic resources. You'll need to build your own GNU toolchain (gcc + binutils + gdb) for cross compiling (don't try to use the one that ships with your OS, it will subtly fail when you don't expect it to), get an emulator (e.g. qemu or bochs) and a bootloader to get started. Do not write your own bootloader code or try to stick to pure asm, you'll get nowhere fast. Use the multiboot protocol so you can easily get booted on emulators and real iron using GRUB2. Note: I assumed you want to work on x86.
In addition to OSdev, the Intel manuals are an excellent resource. They are a pretty nice thing to read even if you don't intend to write an OS. Although reading through them will probably get your OS hacking itch going and you can't stop yourself. That happened to me. The Intel manuals are here: http://www.intel.com/content/www/us/en/processors/architectu... . Volumes 1 and 3a are the most important.
And here are the humble beginnings of my hobby operating system: https://github.com/rikusalminen/danjeros