Well, both 4E Essentials and the Pathfinder Beginner Box are good introductions for a new DM. The PBB is self-contained, but if you have the Essentials books you'll want to make sure you have the following:
- At least one of the Heroes of the Fallen Lands/Forgotten Kingdoms player books
- The Dungeon Master's Kit
- The Monster Vault
If you're missing a DM book or a monster book, you could always download the free
Keep on the Shadowfell adventure from the Wizards website - it has monster stats and contains the material you need for a couple levels. It's a decent introductory adventure but not the best written - if you could find a copy of the Red Box or HS1: The Slaying Stone, I believe those adventures are a bit better designed. If you want to stick with KotS as a starting adventure (which you could do even if you had the DM materials),
the Alexandrian website that Quickleaf linked has some advice for re-working it. Of course, there is
always the option to jump in and write your own adventure, and if you feel confident go for it!
Sadly, there is no free rules reference for D&D 4E like the Pathfinder SRD. Wizards
does have a subscription service called
DDI that includes a
ton of stuff - all the player and DM material from every book they've published, a character and monster building software, and access to the archives of two online magazines with even more!. DDI can be a bit overwhelming for someone new to the game, but it can also provide a lot of value.
If you want to check out Pathfinder more thoroughly, the Beginner Box is an absolutely fantastic place to start. It has some good advice for DMing (no matter what system!) and lots of resources to create great adventures.
Paizo's website has free, downloadable add-ons for the player and GM, and
this blog has converted a bunch of the classes and other rules from the full Pathfinder game into Beginner Box format.
Welcome to the hobby, and may the dice be ever in your favor!