Since you mentioned fork() I assume you’re on a Unix-like system, in which case POSIX threads (usually referred to as pthreads) are what you want to use.
Specifically, pthread_create() is the function you need to create a new thread. Its arguments are:
int pthread_create(pthread_t * thread, pthread_attr_t * attr, void *
(*start_routine)(void *), void * arg);
The first argument is the returned pointer to the thread id. The second argument is the thread arguments, which can be NULL unless you want to start the thread with a specific priority. The third argument is the function executed by the thread. The fourth argument is the single argument passed to the thread function when it is executed.