A way to convert PSD format to something Gimp can read

PNG is a bad choice as it won't support much of the PSD features... XCF is also a bad idea since even the GIMP developers don't recommend it since it is built around the internal data structures of GIMP (which can and do change from version to version).

Lots of features (but by far NOT ALL) of PSD can be converted to TIFF which for example supports layers and some other things.

But to do such a conversion properly you will need Photoshop... it can work with and handle and save layered TIFFs without any problem... PSD has "evolved" from version to verion and has become rather complicated - it can even contain vector data etc.

For a Photoshop tutorial see http://www.adobe.com/support/photoshop/gettingstarted/index.html