If you're doing mixing before adding vocals, you're doing it wrong. Have the song written and finished first. You can't mix and master something that's going to change in a moment. In terms of processing, if you don't know what you are doing and why, don't do it. If you have to ask if a plugin is for the master bus or for individual tracks as well, you're better off not using that plugin and using the tools you know and have a good reason to use.

Also, if mixing is hard to do, you probably have a badly written and/or arranged song. Try looking if you have every track playing throughout the whole song. If so, delete some things for the verse so once they come in the chorus, you get more impact that way.