I'm glad that you're interested in 4e. To date it still remains as my favorite version of D&D. I found that the promise of the Moldvay D&D Foreword was finally realized in the mechanics provided by 4e.
You have already received a great amount of awesome advice on this thread about resources. So I thought I'd give you a bit of advice on "perspective". Particularly because you have never DM'd before. More than any other version of D&D, 4e is particularly suited to "portray" scenes of high-action adventure. The PCs are not schleps that just fell off the apple cart and decided to go adventuring. They could almost be said to be "marked by destiny". Therefore they are more crafty and resilient than the general populace. The general populace could never hope to achieve heroic status. The PCs are heroes from the get-go, and that is by design. When creating and running adventures for your group, keep that paradigm in mind. The patron that sends you on a rat-catching quest is most probably not an appropriate adventure for heroes. Get creative with your adventures and let the players get creative with their actions.
Therefore, when a player says he wants to jump on the back of the gray render and stab it in the brain the DM attitude should be to say "yes go for it", or to roll the dice using the using the chart for difficulty by level, or a combination of both. In other words don't block the heroic actions by assuming they are not "realistic" or could not happen in the D&D world. The chart is your friend, but a "go for it attitude" is very important/necessary.
Describe the action and be evocative, but don't paint yourself into a corner. D&D has a lot of dry mechanics. A game that devolves into "I roll a 16 vs. AC, does that hit?", as the only form of narration, becomes extremely boring. Avoid boring! Remember this is a high-action adventure narrative, keep it as such. By the same token don't create narration that will cause you heartache, as follows:
DM:"The gray render lurches forward swinging it's claws wildly." 20 vs. AC, does that hit?
PC1: Yes
DM: 16 points of damage
PC1: That knocks me unconscious.
DM: "Its claws rend you from shoulder to belly leaving a deep bloody gash, blood pooling at your feet you fall forward unconscious."
That narration is more evocative and better than simply saying, "20 vs. AC, 16 points of damage, you're unconscious." However the catch-22, and what will cause you heartache is when the next round PC2 comes in and says, "Doriok steps forward and yells out to Robrik - get up laddie this is no time for napping! We have beasties to defeat! - Inspiring Word - spend a healing surge + 8 HP."
Make your narration with the thought in mind that the claw rending description should probably be reserved for the time when the PC actually dies (negative bloodied value), not for the time when he goes unconscious. You could have still used something like "Its claws slam into you, knocking the breath out of you. The room starts spinning as you fall forward unconscious." The point is that the description of the extent of the PC's wounds should remain nebulous until he either dies, or he recovers. He could still have a deep gash from shoulder to belly but don't say so until he's actually dead. And if he recovers there is just a superficial scratch instead of a gash. He got knocked out by the impact (force of the blow) not the blood loss and therefore was back up in the fray on the next part of the "scene".
Additionally, I like to DM from the perspective that the PCs are "Action Heroes" in their own movie. My job as DM is to put situations in front of them that are challenging, and let them "go to town" in solving those situations. My job is not to think too hard about fantasy. The mechanics are there to provide a consistent method of conflict resolution, not to explain disparate narration as shown above.
For example, the PCs are barefoot for some reason explained before through narration. In this encounter the room the PCs enter bursts in an explosion that sends shards of glass, and other debris all over the place. The PCs take some damage and are slowed. Possibly narrated as having some of the shards cut their feet. After that encounter it is very possible that the slowed condition will go away. If you thought too much about fantasy, you would be forced to ask why are the PCs no longer slowed since their feet were cut? The real answer is because you, as DM, have deemed that it is irrelevant except for that particular encounter/challenge, and because conditions such as slowed are fine for one encounter but become a bore for a prolonged setup. In other words, the "Action Heroes" can prevail after that encounter without any additional hindrance. The same way that "John Mclane of Die Hard fame" gets his feet cut by shards of glass in scene Y and it hampers him during that scene, but it does not affect his mobility during the rest of the movie. John Mclane took a "short rest" and removed the shards from his feet and semi bandaged them (spending some healing surges to get back to combat efficient form - full hp. But still having some "injuries" in the form of spent healing surges). When you start looking at the game mechanics from this perspective it all starts to fall in place beautifully.
Heroic action does not require loads of combat. One of the criticisms laid at the feet of 4e published adventures is that they were simply combat fests. I think this might be a misunderstanding, somewhat disingenuous from a certain perspective, and a double edged sword.
The 4e published adventures could "appear" to be combat fests because of the Dungeon Delve format used for presenting each encounter. This 2 page encounter format kind of "forced" a combat resolution rather than anything else. The misunderstanding comes from believing that each encounter MUST be solved by combat and that the encounter, as written, must be used. That presentation did not help for alleviating this misunderstanding. Experienced DMs could easily alter the encounters to make combat less prevalent, but inexperienced DMs would possibly not know any better. Therefore my claim of disingenuous. In addition, some of these combats were put in place with what appears to be little consideration for pacing. The initial roll-out of 4e had monsters that had some "issues". For an experienced D&D player the fact that it could take 20+ hit points to kill a kobold was probably an unwelcome eye-opener. This created the potential for combats that would take too long to complete. Without good instructions/guidelines for DMs to handle pacing this could be a real issue. And this is where the problem becomes a double edged sword. The Delve format made it extremely easy for any DM, regardless of their level of experience, to run a challenging/fun encounter. But at the same time it created the artificial notion that "everything/vast majority" of the material in the adventure module was dedicated to combat. If you contrast the delve format to "older" modules where the DM only had a name to the room, sometimes a base description to it, and a comma separated list of monsters you can see the utility of the Delve format. However taking so much space makes it look like combat is the only worthwhile activity.
Therefore you must mix it up. If you look at the successful and exciting fantasy movies you will see a mix of events (encounters), and narration/exposition. You will need to find the appropriate pacing for your group. At times the narration will be higher, and at times the combat will be higher. The important thing is to keep things moving, don't allow the session to stagnate. I recently played in a Star Wars Edge of the Empire game. The first session was exciting and moved rather quickly. It had some combat, some roleplaying and some exposition. It was well balanced and paced. Our second session was true torture. Imagine a Star Wars movie in which Han Solo and Chewbacca spend 2 hours of your, the audience's, time deciding if they should buy a laser turret, or upgrade the engines of the Millenium Falcon. That particular scene should have taken 5-10 minutes tops, but it took 2.5 hours of real game time. Avoid this type of stagnation like the plague!
One thing I have not covered is skill challenges. I'm sure I could write some more on these things but I've written too much as it is already. Above all -
have fun when you DM, be excited and provide fun challenges.
