Go to Settings-Storage and it will display a list of what's taking up your space. The first thing to tap on is "cached data", which will clear the cache and free up that space. The other big space users are usually pictures (backup to Google Photos or Dropbox or any other cloud service, then delete the pictures from your internal device memory) and music - you might need to delete some of the downloaded music from your phone from within your music app. Check also the Downloads file - you can look at the individual files in there and decide whether you can delete any of them. Then consider deleting downloaded apps that you don't use. You can always get them back.