T'is a question as old as time itself: which editing program is the best? Every editor asks this when they first start out. Most editors continue to re-ask this early into their career. The quick answer? None of them is THE BEST. They are all simply tools that are used to get the job done. Not one is better than the other.
So how do you choose one to use and master? Some suggest that you base your decision on several important factors: compatibility, cost, ease of use, accessibility, etc. Most would tell you to use what's available to you right now (free tools like Windows Movie Maker or iMovie). And I think it's incredibly safe to say that an unsurprisingly large number of editors would tell you to just download torrented software. While I'm not telling you which one of these you should do, I will say that these factors will help you decide which program to learn FIRST.
I said "first" back there because I'm about to give you my answer to which program you will choose: All of them.
There was a time (even before I was editing) when you chose your path when it came to editing software. They were all so expensive, difficult to access and took a lot of effort to become proficient in their use. So if you wanted to market yourself as an Avid Editor, that was your choice. And provided you knew your stuff and spoke Avid's language, you could get work fairly easily. There were clients out there who were looking specifically for someone who knew their way around that specific program and would hire you.
Nowadays the game has changed: the cost of software and hardware has dropped significantly, support and tutorials are literally everywhere (in the form of books and online materials), and the client you're trying to convince to hire you is one of two people: 1) They expect you to know the program that THEY favour. If you don't, you'd better learn it. 2) They're not familiar with any of them and couldn't care less which one you use, as long as it's the "right" one. You're the editor, you should know.
I first started on Final Cut Pro 7 (before the dark ages of FCPX) and became very proficient with it. After getting a number of smaller freelance jobs, I was about to secure my first big-paying project with a client. I knew that when they hired freelance editors they preferred them to use the in-house Avid suite. So as I worked to get my foot in the door with them, a copy of Avid got installed onto my computer and I put in time to get familiar with it. One day they called and needed me to start a big editing project the next day. It was urgent. They asked me how well I knew Avid and the answer they got was a confident, "Why, YES I do!"
Shortly after that I was learning Premiere Pro. And then I was saying yes to clients looking for editors who used Premiere Pro. I even took a step back and learned iMovie, which came in handy when someone approached me for one-on-one tutoring lessons using that program. And it took almost NO TIME to master.
What other advantage is there to not devoting yourself to one single program? When the company that owns the one program you've devoted your time to decides to make some drastic development changes that rub you the wrong way, you can easily shift gears and rely on another program that gets the job done.
So if you're just starting out and need a program to learn, don't spend a lot of time choosing, just choose the one closest to you. Then when you've gotten familiar with it, move on to the next one. You'll thank your past self for it someday.