At the simplest level use File > Add Folder to Library and import you music folder. (Press CTRL+B if you don't see a menu bar). However unless you change the default option to copy files to the iTunes Media folder this may end up doubling up your existing iTunes compatible media.
Unprotected WMA will need converting which iTunes can do on the fly during import however you may prefer to edit your import settings before doing so. Thre is no point converting 128k WMA tracks into 256k AAC. They will just be bigger with no gain in quality.
For details on how to have the media available to both applications see Getting iTunes & Windows Media Player to play nicely.
If you have playlists in WMP you want to migrate into iTunes first make sure to disable the iTunes advanced preferences to Keep... & Copy..., export the playlists in M3U format and import using my script ImportM3U.