Welcome to EnWorld! There are a ton of helpful folks on here, so you've come to the right place.
One of my biggest recommendations is to understand the context rather than the content.
What I mean by that is that if you understand the core fundamental principles, the background, and the majority of the major variables, you can tweak and fiddle with things as they occur. In other words, it is better for you to have a really solid grasp on the setting or the locale of the game, the people there, the players and the characters, and any of the obstacles or villains that they might come across. There is a tendency for new DMs to fall in love with his or her own plot, story, adventure hooks, and so forth; and often think of "all of the possible routes." Most of these types of campaigns get demolished in the first setting by the way the party reacts.
A way to mitigate that deer in the headlights effect, if you understand your setting and the overall backdrop of everything, you can ad lib, cut, paste, shift, rearrange, or alter as you see fit. I do this all of the time as a GM, and in fact, most of my best games were ones in which I had a really solid understanding of that particular situation rather than the plot of the adventure.
Now, if you're looking for actual tool tools as Janx suggests, tokens and cards and all of that can be useful. If you're not used to using them, it can actually impede your focus and prevent you from having a successful game. You have to remember, the DM has to essentially captivate and entertain the entire group of players. Surely you have a ton of stuff going on in the background, you have to understand and interpret everything that the party is doing, make rules judgments, and half a dozen other distracting things, but all the players have to go off of is what you say.
That's it. Just what you say. So you have to keep the flow going and keep all of them engaged. If everyone is engaged and enjoying the game, the rules and the tools and what not don't really matter. Now, if you are concerned with things like juggling all of the numbers in combat and keeping balance and all of that, just allow yourself to be somewhat loose.
If you free yourself of the obligation of slavishly following the rules, especially in more complex systems, you can allow your narration to shine and cinematic events to occur. If you are a real mechanics-heavy crunch guru (which I can be and a lot of DMs are) then try to be as organized as possible. If you're playing D&D4e, you might want to check out Power2ool, which is a free site which is very nice, or even DDI which is worth the 10 bucks a month if you play often.
All I use when I DM, even with a system I'm not particularly familiar with, is a set of notes (usually bound in a three-ring binder) and a stack of index cards denoting certain things. Sometimes a card might just have one word, like, FEAR, written on it, to consistently remind me of the tone that I'm trying to maintain.
But, a lot of my playtest GMs like to have a sheaf of all of the party's stats on them with transparent sleeves over the paper. This allows them to write on the plastic as necessary (this is also useful for players to keep track of stuff) and take notes/list effects etc.
To sum up: be organized, know your setting/context, and try to maintain engagement rather than confining within the rules.
Have fun, and remember as a young fellow from the Lodi High School D&D club stated, "If you're not having fun, you're losing."
![[OMENRPG]Ben](/data/avatars/m/6677/6677983.jpg?1565264961){kind=avatar}
