The C standard says that there is very little difference between the #include <header.h> and #include "header.h" notations.
[#include <header.h>] searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the \< and \> delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined.
[#include "header.h"] causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the "…" delimiters. The named source file is searched for in an implementation-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed as if it read [#include <header.h>] …
So, the double quoted form may look in more places than the angle-bracketed form. The standard specifies by example that the standard headers should be included in angle-brackets, even though the compilation works if you use double quotes instead. Similarly, standards such as POSIX use the angle-bracketed format — and you should too. Reserve double-quoted headers for headers defined by the project. For externally-defined headers (including headers from other projects your project relies on), the angle-bracket notation is most appropriate.
Note that there should be a space between #include and the header, even though the compilers will accept no space there. Spaces are cheap.
A number of projects use a notation such as:
#include <openssl/ssl.h>
#include <sys/stat.h>
#include <linux/kernel.h>
You should consider whether to use that namespace control in your project (it is quite probably a good idea). You should steer clear of the names used by existing projects (in particular, both sys and linux would be bad choices).
If you use this, your code should be careful and consistent in the use of the notation.
Do not use #include "../include/header.h" notation.
Header files should seldom if ever define variables. Although you will keep global variables to a minimum, if you need a global variable, you will declare it in a header, and define it in one suitable source file, and that source file will include the header to cross-check the declaration and definition, and all source files that use the variable will use the header to declare it.
Corollary: you will not declare global variables in a source file — a source file will only contain definitions.
Header files should seldom declare static functions, with the notable exception of static inline functions which will be defined in headers if the function is needed in more than one source file.
static) inline functions.