As a new GM, (and relatively new player), start at level 1, Learn the game from the ground up.
Leveling in real RPGs is real time slower than CPRGs. I suspect that its could be a similar amount of hours spent, but you log 12 hours in WOW more easily than you can in D&D. This supports my basic XP rule: hand out less XP for more frequent gaming sessions, more XP for less frequent sessions. People like to see change, and if a session is 4 hours, playing every week, you might not mind leveling up every 4 sessions. Playing once a month, that would mean only gaining 3 levels in a year. That means you don't feel like you accomplished much. So you pad the XP to make the players feel like they're gaining something. It's a psychological thing.
D&D is not a CRPG. There's not a literal entity that exists as a "quest giver". There's no such thing as a quest. These are all constructs we fabricate. The sandbox guys would probably more strongly agree with that thought. In my "I write up an adventure ahead of time" model, I make up a problem. Usually a bad guy with a plan, a location, minions, and put them in action. I then work out how many different ways I can make the bad guy's stuff intersect with the players in a fashion that will INTEREST them.
That means I don't use a blunt instrument of the "high level NPC who asks the party to stop the bad guy." Instead, the party runs into a caravan of refugees who are fleeing their diseased village. NPCs known to the PCs might talk about rumors of the place. NPC patrons of the PCs might ask them to look into the matter, or reveal information that the bad guys actions impact the PCs. For instance, if the players own stock in a tea trading company, it turns out, a profitable flavor of tea comes from that village. He's losing money every day that village is empty.
The point is, there's not quest givers, there are plot hooks. You want your plot hooks to be something the PLAYERS and PCs would be interested in pursuing, not being coerced into.
What I find works, is that the first session I bring the PCs together (in bar for example), and then bring on an event that affects the whole party and gets them into the action. Sweep them up in a problem that directly affects them (the village that the bar is in is under attack by orcs). No quest giver, it's an event that crosses their path. This gets them MOVING, and they will then drive the action, by reacting to what I set in motion. From there, the first adventure is stopping the attack (which might actually just be a raid), and then they will probably track down the raiders, which can lead to a small dungeon crawl. They find the head orc, fight him, game over, hand out XP.
Then you ask the players what they think they will do next. You write the next session based on their declarations of intent, mix in some complications and some bad guy shenanigans that have nothing to do with the PCs but crosses their path anyway, interfering with their original plan. From that, you have a formula for making adventures based on what the players want to do and mixing in your ideas for stories as well.
PbP is slower than molasses. Combat shouldn't be. Watch how others do combat in PbP and pick the methods that work the smoothest and fastest. Also, avoid doing as much combat. You can't literally stop the PCs from attacking anything, but you can avoid time wasting combats. Skip the mook fights and random encounters. PbP shouldd have more role-playing, with sword play being the climax with the villain.
Stopping the random violence by PCs. Step one, filter out the PCs during char-gen. Tell them you are running a heroic campaign, no evil PCs. Evil PCs are less predictable, whereas a good PC will almost always rescue the princess when presented with on that needs it. Step 2, have consequences for random killing. Namely, the law. The difference between having every NPC be high level to protect themselves from PCs, and sending higher level guards after PCs when they kill low level PCs is carrot and stick. With every NPC being tough, it is an arms race that you have to take ridiculous precautions for, even for a carrot farmer NPC. Using the guards as stick method, you get to make your NPCs realistically. Yep you can easily kill that farmer. But if you do, the cops will hunt you down. Just like real life.
That's just a few things...