file sharing from imac to windows

Hi,please can anyone put in simple form how to see my imac files on a PC.I can see my PC files on my imac and see the imac on my PC,but when I try to open them I get asked for my windows credentials.I have tried every user name and password I know.Still no luck after many hours.Imac is running Yosemite and PC is running windows 8.Thanks in advance.

Thanks for the reply.I have indeed.I think I must have exhausted everything else also.I got so frustrated with that windows asking for network credentials caper.I installed bootcamp today.That showed it who was boss.