This is important: the recording must be in good quality without any background noises.
There are several factors when you create a song that matter when it comes to quality. The most important being the actual recording situation: no noise, crystal sharp instrumental recording, sampling, careful mixing, excellent mastering etc. It is hard to over estimate the relevance of this to create a great track!
Ideally you upload a .wav-file or FLAC, sample rate at 44,1 kHz, stereo and 16bit. You can upload the file through Google Drive, Dropbox or your own phone.