Hi Sega,
I was the same as you when I first started - I felt like I was trying to shoe-horn my research into any passing epistemology! Can I recommend Crotty (1998) The Foundations of Social Research as a starting point? It is very easy to read and should make things a bit clearer for you.
It also depends on your methods of course, some methods go hand in hand with particular epistemologies (e.g. ethnography and interpretivism). It's good to think about it but don't panic early on about "making it fit" - as your study unfolds and you become more experienced the right answer will come to you - I certainly found this to be the case for me. The other thing I can really recommend is doing lots of reading around your study and find out what methods other people have used, and what their epistemological position was, again this helped me no end when pinning my study and approach down.
I hope that helps :-)