Go to Edit > Preferences > General > Import Settings . Choose Encoder: Set "Import Using" to AAC Encoder .
If you have existing files in other formats (like CD rips) and want them in .m4a : itunes-m4a-download-blog
Select a song in your library, then go to File > Convert > Create AAC Version . Quick Tips for M4A Files Go to Edit > Preferences > General > Import Settings