Dungeon Master, Game Master, DM, GM, Referee, Castle Keeper, Labyrinth Lord. Call them whatever you like, they are the driving force behind the game, and without them you just have some people sitting around a table and making some shit up without benefit of arbiter or focus. Someone needs to be in charge. It's the nature of the beast, and while I know that we have genetically modified a few of these creatures to run without the benefit of said 'head-man', mostly, we have em.
Not all players aspire to eventually, one day, god-willing, become the DM (I'm gonna use DM because i'm old and that's what I say. I don't care much for any of the other terms, and for me it applies to all games that I run). Some players are happy to forever be the 'character', and that is absolutely fine, but I think that it's pretty safe to assume that most players eventually get the urge to take charge and run a scenario or campaign of their own creation. It's important, because if that never happened, the pool of DMs would run dry really quickly.
When I first ran a game, I had absolutely no clue what I was doing. For the sake of perspective, it was around 1982, and I was playing a mashup of Basic/Expert and 1e. I mostly knew the rules that were available, and I had played enough to manage my way through a combat and read from the boxed text of a pre-written module. I knew what all the dice were and I had the jargon down pat. Running the game was thrust upon me, so I had little choice, but I did my best and in the end I think everyone (3 of us) had a good time. I really loved weaving the story, playing the NPC's roles, and attacking as the monsters...it was really exhilarating and felt uber-creative.
I want to be very clear on what I say next.
I was scared shitless, felt completely out of my depth, and I had no one to turn to for support. The books, the dice, and the looks on my players faces were my only guideposts. There's a good chance I bent or broke a ton of rules. Monsters died, characters died, dungeons were looted, pizza was eaten, and in the end I survived. Over time I guess I became a better DM, mostly through trial and error, and occasionally being exposed to other people who ran games. Back then we had no internet, so the only resource we could tap was one another, and maybe Dragon or White Dwarf magazine.
The point of this is simply to say, if you want to DM then DM. While there are plenty of people playing the game who will offer very specific advice on how to run your game, which rules are the best and worst, and what your function as the DM 'actually' is, in the end none of that crap matters. Just DM. Run your game and have fun. If you want to seek advice or direction then by all means, do so. There is no end of people who are willing to help (or hinder) you on the internet. You can listen to all, some, or none of them. It doesn't matter. You have a story to tell and a game to run/play and those are the only things that really matter. You will learn best by doing.
As a player and DM I am always looking for new resources, opinions on the game, ways to organize, and seeds for stories. I do so KNOWING that there will be alot of noise to filter, but I think that if you are 'coming up' in the hobby today, you may have a hard time filtering out some of that chatter. To that end, i'd like to offer up the places I think you can get some decent information.
1. Play in games with other DM's. You may have done this a time or two and now you think you wanna be the DM, but you can do both. Keep playing and when you play, determine what you like and dislike about how the DM runs his/her game.
2. If you are going to use the internet as a resource, try to follow/find resources that people you already know (online or off) use and can verify. The caveat here is that 'just because everyone is doing it, doesn't make it right', so as always try and draw your own conclusion after reviewing a resource.
3. The sourcebook material of the game you want to DM should be your primary guidepost for running that particular game. If you've read a text many times and still have no clue as to how it should work with you in the DM's chair, it's likely not a great game to play. Good source material is aimed at both parties, or broken into separate texts aimed at the appropriate party, IMHO.
Do I use any online sources, or watch any games being played? Sure.
I really like the community currently going over at the DMSupportgroup and I'm a somewhat active participant there. There is a nice mix of noob and grognard over there and they have some nice resources for new or aspiring DMs. I suggest taking a look. I'm an OSR guy, and we are in short supply there as there tends to be a focus on 5e, but I do my best to throw my Old-Head into the fray and make OSR advice available. I'm not always right, BTW, but my opinion is as good as anyone's I imagine.
Being a follower of folks and communities I have an interest in has helped quite a bit. I find that G+ has many good, very focused communities, and they are often overlooked in lieu of Facebook or Twitter. I'm a member of the OSR G+ Community and several others where some wonderful conversations about game-play and mechanics take place.
Sometimes, if i have some time to kill, I will watch some actual play vidcast or maybe listen to a podcast, but very few of these keep my attention very long. I know that Critical Role is a big deal right now, but I honestly find it uninteresting. I have watched a few of Dan Harmon's HarmonQuest and I enjoy that quite a bit, but these are run in a very tongue-in-cheek manner and while fun, they aren't how I run my game.
If you have some good resources to add for old, new, or aspiring DMs, please drop them in the comments below. I'd love to see a growing list for folks to access.