The MSDN article and will do a better job of explaining than I can, but the summary is:
First you need the following in your manifest file:
<Capability Name="removableStorage" />
and at least one file type declaration (these are the files that you will be able to access on the SD card:
<Extension Category="windows.fileTypeAssociation">
<FileTypeAssociation Name="foo">
<SupportedFileTypes>
<FileType ContentType="foo/bar">.bar</FileType>
</SupportedFileTypes>
</FileTypeAssociation>
</Extension>
Then in your code to get the root of the SD card use:
StorageFolder sdCard = (await KnownFolders.RemovableDevices.GetFoldersAsync()).FirstOrDefault();
if (sdCard == null)
{ return; /*no SD card*/}
and to enumerate the folders use:
IReadOnlyList<StorageFolder> folders = await sdCard.GetFoldersAsync();