Windows DLLs

Where Can I find winmm.dll documentation ? (or other dlls)

The most productive technique is to read the documentation of the functions you need to use and it will tell you what DLL to use. If you list the functions in the DLL, then you still must read the documentation. Readding the documentation won't help if you are not yet familiar with the basics of Windows programming, and use of the functions in winmm.dll will be quite difficult without reading and understanding the documentation.

How can I find out what functions are within a windows dll when i want to use it?
Thanks.

There is much to learn about Windows programming. If you are learning and using C# you should try to use .Net and classes designed for .Net. If you must use the Windows API and other DLLs directly then look for sample code. If you cannot find sample code then you need to spend a lot of time learning; many months.
 
If you need help using winmm.dl, then you probably should not use it; you should use something designed to be used by .Net.