My suggestions:
- As others have said, nothing you read is going to mean much without some experience. Find some players that will put up with mistakes, often hilarious, party-killing mistakes, and go.
-- If you must read outside things, start with the Angry GM mentioned earlier. Some of his recent posts may be an even better introduction than his book. He isn't always correct, and his tone is an acquired taste, but he does the very important step of telling you on each piece of advice why it is there, so that you can make an informed decision about how to adapt it to your game, and breaks it down into usually clear steps from the ground up. If you must read Robin Laws, only do so after reading others and with some practice, because most of his advice is the complete opposite of the Angry GM's, and is often feel-good pablum rather than practical advice on how to do anything. Even when Laws is correct, it is situational advice for an experienced GM, not something you can just do and understand later.
-- And, in fact, starting as soon as possible is what the Angry GM tells you--here's a way to get started, go run a game doing these simple things, then come back and read more. Whether his exact suggestions for how to start are the best or not doesn't really matter, though the one to do a one-off with pre-gens is gold. It's always a good idea to learn a new system with something that you aren't 100% committed to doing a campaign for. If it happens to turn out great, you can always flesh it out into a longer game later. But there is no promise of that.
-- There is a difference between all the discussion, rules-interpretation, and thought between sessions versus what you do in the session. In the session, just do the best you can to be focused on keeping the game moving right now. If it completely comes apart, and you must do more than make a quick ruling, or maybe you are overwhelmed, then explicitly stop the game for a few minutes, resolve things best you can with the group, and then explicitly get back into the game. This is especially important if the players are more experienced than you. When you drop out of game that way, you are deferring to their experience to help you learn something that, in your judgment, you really need to know right now. While learning, you want to make it explicit because this will help you get into the proper frame of mind, will be clear to the players when you want to discuss versus keep going even with nasty mistakes, and will help you avoid the habit of running for help every time there is a little uncertainty. Going for help needs to be a conscious choice. The cost of stopping the game isn't all that great, as long as you don't do it a lot, but it is a cost.
- I have yet to see any game that does a better job than Toon of getting the GM started, If you are really nervous about it, buy that and run it. It's easy to learn, short to play. You won't learn everything you need to know to run D&D or something similar running Toon, but you will get some of the basics out of the way. Barring that option, it would be difficult to top Moldvay Basic as a good starting place. Not perfect, but then nothing is. I don't own OSE, so can't comment on that, but as I understand it, should be compatible with Moldvay. If it lacks the how to start advice of Moldvay, you could run OSE the Moldvay way. If it's got its own version of that, good enough.