Where do I need to place code for a rollover button?

What I’m trying to do is place a fairly large call to action button at the top of my homepage. I have created the image sprite, the html index file @ css stylesheet which are all located within a single folder. What I need to know is where exactly do I place the folder within my site? how do I call the code once it is in place? and how do I position it within the page ( I need it floating over another image on the top portion of the page just under the header)
I am extremely green when it comes coding so you might have to dumb it down somewhat. Thanks a million in advance, I look forward to a solution to my dilemma. Cheers;)