MacTex will automatically search for files in your "personal" part of the TeX directory tree system. Assuming your userid on your Mac system is "aku", the root of this personal part would be at
/Users/aku/Library/texmf. MacTeX's installation routine should have already created this folder -- as well as several subfolders such as
bibtex, and probably a few others -- at the time it set up all other folders. Since your aim is to store the file
mypackage.sty in a texlive/mactex-compliant way, I suggest you put it in the folder
mypackage will need to be created by you, obviously, before you can save the file
mypackage.sty in it. Because you'll probably be installing other packages as well in your personal part of the TeX directory tree over time, it's advisable (but it's not required!) to create separate folders for each LaTeX package. Happy TeXing!