1. There are many linux distros for many purposes; as always your usually recommended to try em out and see which one your the most comfortable with/ is more fitting for your needs. Take BSD variations for instance. Open-security, Net- portability, Free- performance.

2. BSD is closer to Unix than linux although slackware is closer to unix in terms of linux distros. I personally use BSD (free, open) and slackware. Slackware to me gives me more control over the linux system and the syntax is similar to BSD which is closer to Unix in nature so the transition between the two is essentially easier for me plus your getting to know the ins and outs of each system.

3. Knowing/ learning the command lines is the best way to "master" the system (though you'll never truly master Unix). There are visual/ graphical interfaces (xfce, fluxbox, blackbox,<--lightweight | heavyweight --> kde, gnome) you can use which may make things a little easier in terms of maneuverability. You will not memorize/ know every single command line, but the more you work with your system, the more comfortable you'll be with it, hence certain commands will become a use of habit (just like learning french, spanish, etc.)

4. The handbook, forums, search engines are your best friend. You will also make several mistakes/ run into a few walls. Like anything in life it is a learning process and will make you stronger in terms of understanding the system. Though you dont want to make any critical mistakes, making mistakes will benefit you in the longrun because you will know what to and what not to do if the problem or similar situation occurs later. Troubleshooting and common sense are part of the "mastering" the system. I always recommend learning the system from the ground-up as well. C/C++, perl, ruby, python, etc. are additional essentials. Some dont recommend it (for one reason or another) but learn asm (assembly) to really get to know the grindworks of your system.

5. First, back up ALL of your data from you OS before installing. Like Corona stated, its best to do it on an extra harddrive. Get the handbook and follow the instructions. The are concise, but mistakes will be made, so ask for help, no question is a stupid question (unless its OpenBSD , but its usually a point of figuring it out yourself and following the instructions with greater precision). You also have other forums based on what OS you are using. Whatever mistake you made, it has been done before, so do some research before asking.

6. The time and dedication you put into learning the system are all factors of measuring ones skill level. Depending on your learning curve, there will be points in time in which you will want to give up, but continue to have the determination and will you have now and you will continue to grow. You will know when you reach certain levels of certainty based on the amount of comfort you have with your system, but as I said before, you will never truly master the system, there are always new things to learn..ALWAYS.
Depending on what systems you become comfortable with, search around (amazon, etc.) for recommended literature to get to know your system from novice to pro including recommended UNIX literature. Best of luck to you and welcome to the free world my friend. You have made the first step toward your intellectual freedom.

---------- Post updated at 10:35 PM ---------- Previous update was at 10:07 PM ----------

One last thing (idea), now this isnt for the light-hearted, but if you really wanna be a total bada$$, use lfs (linux from scratch), but I would recommend what I and Corona said first.