You've decided to run a role-playing game, and you invited a handful of folks to join you. Whether you've written your own, or borrowed someone else's, there are some things you should consider as you go forward.

Have a regular run time. Trying to do ad hoc scheduling will likely cause you to lose your most reliable gamers, and will lead to inconsistency.
Pick a sane amount of time for your group. For some people, 5 runs a week for 8 hours at a time is reasonable; others find a weekend day for 12 hours is good; yet others approve of a 6 hour run once a week.
Have a food plan.
House Rules
Almost no one uses only the published game rules. You are going to have house rules. You should keep a collection of house rules, so that everyone knows what they are. Additionally, you should have a meta-rule, which specifies how you will add new rules. I recommend, "The GM will make rulings on the fly. All rulings hold true for the remainder of the run. Rulings may be edited between runs."
In any run, you should have a number of encounters; they may be visits with shopkeepers, or battles to the death with some orcs. You should prepare cheatsheets for encounters, so that you can run them very smoothly. Having a half-sheet with all the relevant information can make these quick, and prevent boredom for your players.
Dynamic Scenarios
Many GMs tend to run encounters "by the book" -- having NPCs effectively never moving (so player can retreat, rearm, and come back into the same encounter). NPCs should be adapting - if they're attacked in one place, expect them to build up some defenses.