Build a simple HTTP server in C

I suggest you take a look at tiny httpd. If you want to write it from scratch, then you'll want to thoroughly read RFC 2616. Use BSD sockets to access the network at a really low level.

I'd recommend that you take a look at: A Practical Guide to Writing Clients and Servers

What you have to implement in incremental steps is:

1. Get your basic TCP sockets layer running (listen on port/ports, accept client connections and send/receive data).
2. Implement a buffered reader so that you can read requests one line (delimited by CRLF) at a time.
3. Read the very first line. Parse out the method, the request version and the path.
4. Implement header parsing for the "Header: value" syntax. Don't forget unfolding folded headers.
5. Check the request method, content type and content size to determine how/if the body will be read.
6. Implement decoding of content based on content type.
7. If you're going to support HTTP 1.1, implement things like "100 Continue", keep-alive, chunked transfer.
8. Add robustness/security measures like detecting incomplete requests, limiting max number of clients etc.
9. Shrink wrap your code and open-source it :)