I normally start it out kinda cheesy and loosy-goosey with the rules.
Make sure people are playing what they can enjoy and do some cliche stuff to start it out. And If it comes down to it and you made your crawl to hard (or easy) Just fudge some numbers to even things out (ac and to hit numbers work quite well for this)
From there...Well see how you grow your world. Nothing we can say that you won't learn in the first few sessions as a DM.