Which songs am I allowed to upload according to copyright law?

You are allowed to upload all songs you have all necessary rights for or if the rightsowner assigned to you all relevant rights for digital distribution (best with a written contract).

In any case the upload is allowed if you have composed, written, recorded and produced everything on your own and did not use already existing, trademarked recordings from others.

Tip:
If you have composed or written songs in collaboration with other musicians or as a band, clarify copyrights before you upload the song. You avoid subsequent disputes and litigations.

When you upload third-party compositions and cover versions, the original authors, composers and publishers must be credited correctly in the metadata fields. The royalties are distributed by the national collecting society. Attention: Since a lot of cover verions are actually edited versions of the original, you absolutely need to get permission from the rightsholders first (e.g. lyricist, composer or publisher).