1. How do I create a master file?

2. If I use the Print Screen button to take a screenshot, can I still upload it to the Steam Workshop? And, if so, how do I do that?

The Creation Kit does not support creating master files. You will have to use a 3rd party tool to convert an esp to an esm, I believe TesVEdit is the preferred one for Skyrim. Most mods don't need to be master files though and will only make compatibility problems. I'm pretty sure the CK doesn't let you edit a master file either so you will have to keep a backup copy to make changes on and then reconvert that each time. What are you trying to do that you think needs to be a master?

When you go to your mod's page in the steam workshop it will have an "add/edit images & video" link. That will give you an option to upload a screenshot from your computer to use on the workshop.

As far as I know, creating a master file is pretty easy: just change the extension esp into esm. As for your second question, I don't understand it; I don't see how taking a screen shot would prevent you from uploading your plugin to Steam... unless it's the screen shot you want to upload? In that case, upload your plugin and once it's done add the screen shot to it when you're asked to enter a description.