Whenever you install or remove software, be sure to read through what's being removed. You don't want to accidentally uninstall something important. This is very unlikely to happen with official Debian packages, but you should be especially careful when installing packages outside of Debian's repo, as they may not be fully compatible with your version of Debian.
In any case, I'd log in to a tty (ctrl-alt-any function key) and install whichever desktop environment you had before using apt.
You can start anywhere you want! I often recommend starting with Star Trek: The Next Generation, since it's aged a little better than the original series. You might prefer to jump ahead to season 2 or 3 to get to the really good stuff, but even season 1 is worth watching.
Up until Enterprise season 3 it's pretty much all episodic (or in DS9's case, mostly episodic with a subset of the episodes forming a series-long story arc), so you can pick a random episode or movie with a cool-sounding description and start there if you want. That's how I got into Trek, just picking random TNG and Voyager episodes.