How to create a C++ DLL API ?

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C++

Hello I'm a little bit nervous about this..
umm
i want to know how i can create a DLL(vcPlayers.dll) that contains some functions for example
this function is on the dll :

void register_user(char *name)
{
printf(name);
INDEX_NAME = len(name);
}

and i have another DLL(vcOnline.dll) that is injected in a game (WildShark.exe)

i want to use that function(register_user) from the DLL(vcOnline.dll), how can i do that?