next step , ie mastering
typically ones uses some combination of compression, limiting, and EQ for Mastering; you would use these effects on the Master Track (shift-⌘-m).
note: Mastering is an art form, one that takes a lot of practice, and not something that can be taught in a single forum post. do a web search on "audio mastering" and do some reading
save or save as , what’s the difference ?
once a project has been saved for the first time, Save saves the project you are currently working on to the file to which it belongs. Save As would create a new file with your current project, saving your current changes to it.
IOW, the very first time you use Save or Save As on a project, they perform the same function, after that, you are either saving your changes, or creating a new copy of your project with your current changes (good for incremental backups)