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.

Scheduling
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."
Encounters
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.