By "Hello, World!", I am assuming you mean a HW boot loader or OS stub, rather than the general question of how to use C. If it is the latter, then I would recommend moving this inquiry to the General Programming sub-forum.
As jmziprick stated, you should review the Bare Bones example in the Wiki. However, I would go further and give you a number of links to read (if you haven't done so already), just to get you up to speed on what you will need to know when working in the OS dev space:
Getting StartedHow To Ask QuestionsFAQRequired KnowledgeBeginner Mistakes (the "deadlines" section in particular)
What order should I make things inCode ManagementHow kernel, compiler, and C library work togetherUsing Programming
Languages other than C
Real Mode, especially the section on memory addressing, and
SegmentationMemory Map,
Detecting Memory and
A20 LineBIOS, and
Boot SequenceInterruptsBootloader and
Rolling Your Own BootloaderFAT and
SFSWhile this is a lot of reading, it simply reflects the due diligence that any OS-devver needs to go through in order to get anywhere. OS development, even as a simple project, is not amenable to the Stack Overflow cut-and-paste model of software development; you really need to understand a fair amount of the concepts and principles before writing any code, and the examples given in tutorials and forum posts generally are exactly that. Copying an existing code snippet without at least a basic idea of what it is doing simply won't do. While learning itself is an iterative process - you learn one thing, try it out, see what worked and what didn't, read some more, etc. - in this case a basic foundation is needed at the start. Without a solid understanding of at least some of the core ideas before starting, you simply can't get very far in OS dev.
Hopefully, this won't scare you off; it isn't nearly as bad as it sounds. It just takes a lot of patience and a bit of effort, a little at a time.