If I'm not wrong, you want to decompile a DLL which is already compiled and obviously you don't have the source right?
Correct me if I'm wrong but there's no way you can have the full source of a compiled library. One thing you could do is reverse engineering on the DLL and see if you find something that can help you.
If you're interested on reverse engineering you could start by downloading OllyDbg and read some info, RE it's not an easy job.
And if you really want to disassemble a DLL you'll need a lot of knowledge in ASM.