Hi and welcome to this community!
Im going to make my best to help you.
To start somewhere, please begin to check which quality you are streaming your music with. Go to settings, music quality and then choose normal. This will lower the data usage. Also please uncheck "sync over mobile network" if that option is turned on.
If you still feel like Spotify uses a lot of data, please reinstall Spotify to see if it helps.
If my post solved your issue, please accept it as a solution.