For the sake of your game: read, re-read, test, and re-re-read times ten the rules and rationalizations behind Challenge Ratings and encounter rewards. If you dont, one of these things will happen:
1. PC's wax every creature without breaking a sweat and get bored.
2. The first encounter kills 3/4 or more of the party.
3. The encounter goes great and you give away a magic item you think is cool, which then causes #1 infinitely, till you take it away and the players bitch.
4. The encounter goes great but you give meager treasure, eventually leading to a brutal #2 during some climatic showdown, cuz the party never got stronger.
Secondly, No matter what the monster manuals say or even your own notes, the attacks, damage and hit points of enemy mobs can be whatever you need them to be.
Third, a great final encouter is when everyone in the party is on there last leg and somehow manage to eek out the final victory, desperately praying for a great roll as they do. See my second point. Bad guy bosses should have no more and no less HP/Magic/Attacks/Dam/etc then necessary to run the party through the ringer.
DO NOT LET players go beyond the core rulebooks for D&D (players handbook, MM, DM's guide) for their characters feats and such until YOU have a handle on the basic system and rules.
Fifthly, dont be afraid to just wing it and say to the guy who wants to jump off the balcony, grab the chandelier, swing across the room, sumersault through the air and hit/kick/stomp some baddie from behind. Just say "roll a d20" and make a quick judgement. I can guarantee you there are no rules ever written for any game to honestly adjudicate such actions, and all games are chock full of those kind of actions. Also, its fun to see the players face when he rolls great and pulls off the manuever only to back that up with a "2" for the attack roll, and still ends up on his/her ass. They will sit there and think, "damn, I just blew a perfectly good roll for nothing!" Its sweet, truly sweet. MMMMMMMMmmmmmmm. Good players will realize the tactical problem and conserve the number of rolls for success, allowing you to avoid the hasty issues like that. GREAT players will keep on doing it anyway cuz its fun and makes the game move and when they pull it off everyone remembers. No one remembers the guy who just always went conservative.
Sixthly, if the players havent ever seen an GazeeboOrc, and they meet one, dont say "You see an orc gazeebo". Say something like "You see a monstrously large muscular humanoid standing across from you. He smiles at you, picks up a rock and camly pulverizes it do dust in his fist in a display of arrogant power. For some reason, he looks happy to see you." Trust me, it will change the whole freaking encounter.
Lastly, ignore everything I said, and just play and learn and take your lumps. DMing is a craft one has to develop. It gets easier with time and you will find your own feel, voice and flair. DONT try to be another DM.