I know I sent this to you already, but for reference on the forum:
I think it helps to keep in mind that UNIX commands are text-centric; that's kind of the philosophy (at least, to my knowledge). Many commands will be based around creating, editing, and searching through text files. You can look more into this independently. I'm sure a better explanation of the reasons behind, and consequences of, this property are discussed in any major reading on Linux!
I think you'll demonstrate basic proficiency if you learn some basic filesystem manipulation (cd, pwd, ls, mkdir, cp, rm), remote access (ssh, scp), output-chaining ( >, >>, | ), and text editing / searching (echo, cat, grep). Also, know man [COMMAND], which gets you whatever documentation is available for a given command.
There are a bunch of other helpful commands that you will learn as you use UNIX-based systems more... But you only really learn them because you need them! I've been using Linux for 7 years, and I don't know much more than my friends who've been using it for less time. We all just use what's necessary!
To practice, I'd spend some time navigating the file system using command-line. Then try to create, edit, and search through some text files. IMO, it doesn't take too long to get the basics down. I think you can do it in a day if you're vigilant. Like most things, repetition helps, so don't just do it once and say "got it", even when the commands are simple.
Oh, and learning the acronyms can help a lot; the names can be pretty self-explanatory! For example, pwd = "print working directory". Guess what that does?
Good luck!
EDIT: Whoawhoawhoa, sorry! I missed another big ticket topic in Linux: privileges. Understand what drwx-rw-rw- means, or -rwx-wx-wx, or any of those variants. Also learn about chmod (which you use to set privileges), su, and sudo. Know what root is. These are also incredibly important to understanding Linux commands.
Also useful are those universal directory calls: ~ is home directory, . is current directory, and .. is parent directory.
It's also nice to know about variables and path (great examples of the text-centric nature of Linux). Try echo $SHELL.
Finally, if you want to just know about Linux, learn about the basic file structure (what's usually in /bin or /dev or /usr or /home, etc.)
Phew. Now I'm at least missing less.