The MP3 format, however theoretically (and really for the hardware manufacturers) submitted to a licence, is nowadays a real standard for pop music, and is to get so for classical music and Jazz (cf Deutsche Grammofonen shop 320kbps). Thereby, it seems mandatory to propose this format, except for audiophile uses.
Yozik also lets you sell your music in wav (Artist and Label accounts only, regarding the necessary disk space for an album - an approximate 400MB for one album -). The wav format guarantees a CD quality sound.
Other existing formats are bound to be dealt:
FLAC format compresses the music with a coefficient of 2, without any musical loss, and has the advantage of being open source. Most players on the internet permit the FLAC files play, after the installation of an appropriate codec. For Windows Media Player:
If you propose the FLAC format, we advise you to give also the link to the codecs for the main players.
The ogg format, that’s also open source, has a better quality than the mp3, but is less and less recognized, in particular for the cellphones.
The AAC format benefits from the iTunes’ popularity, and gives an excellent compression quality, but is ignored by the walkmen and the cellphones compatible with Windows.
The mp4 format is not democratized and recognized by walkmen and cellphones yet. It’s used above all for videos. It’s however the future format for the audio.

Yozik can take charge of the encoding for Artist and Label accounts, from a CD sent to us, for 49€ TTC for one album (we verify manually each encoded track).
To encode it by yourself, we advise you to use Audacity, an open source software permitting to do almost whatever one wants with music files. Verify that you always encode from digitalized tracks at a sampling frequence of 44100Hz, because this is the only frequence compatible with computers and mobile phones.
However, a tutorial shows you how to easily encode your tracks with iTunes or Windows Media.

Choose the ID3 v2 or v1 because these are the most compatible standard formats.
You can also add a cover in your tag with different software such as iTunes, but it will be seen only in this player. It will also weighten your file.