1. Find a balance between Rules and Story
Be a STORYTELLER not a gamemaster. Your players can kill monsters in a dozen days in dozens of different computer games. They do not need a tabletop GM for that. However, there is no cpmputer game (and probably won’t ever be) that can create a customized, unique story for a group of varied characters.
That said, you cannot place too much emphasis on the story. If an entire session goes by without a single dice roll or other conflict resolution, consider what the world of your story must be: a world which allows or denies the actions of the characters without regard to the abilities and failings of the characters. The GM may allow an automatic success or failure on occasion, but it should not be the rule. Otherwise, the story is the GM’s story alone, wherein the characters (and by extension the players) are simply rats in a maze.
To summarize, use the rules. Follow them most of the time. The better you are as a GM, the easier this will become. If you NEED to bend the rules for the story, you may. But, recall that rules exist to keep things fair. The more you bend the rules, the more likely it is you pander to or control your players.
2. The best stories are character driven
It is not your story, it is THEIRS. You may devise a story arc about the solving of a crime or the fall of an empire. But, your audience is your players and the story lies not in the what but the how, who, and why.
Encourage your players to create characters with goals and personal issues to resolve. As a storyteller, you can create obstacles. Ideally, use the realistic and reasonable obstacles one would expect to encounter pursuing the goal. You can also tie personal goals into larger story elements or other player character goals.
Example: Simon the scribe is looking for a stolen family heirloom. Gerard the knight errant seeks revenge against his brother’s killer. Easy story tie-in: the thief and the killer are the same man. In this way, the player characters are drawn together toward a common goal and can still contribute to the pursuit in their own ways.
LISTEN TO YOUR PLAYERS! They will tell you what they expect, what they want, how things are going, and more. They will do the work for you if you learn how to let them. If you don’t seem to hear it, ask bluntly and directly after the session.
3. Plan a good beginning, the rest will follow
Plan the beginning well to reach the end well. You may guess what your players will decide in a particular situation but no one can can (or should) plan every detail of every session. Instead, plan the campaign, story, and session beginnings. Rules-of-thumb, a set of pre-made multi-purpose characters, and a willingness to adapt to a new situation will do more than several hours of session planning.
Think carefully about what type of story you want. Ask you players about it, too. Consider what characters are best for the story. Then, plan how that style is best executed. If, for example, you are looking for high adventure and swashbuckling, you would consider what technology, environment, and culture (etc) are most appropriate. The Pacific theater of WWII would likely not be a good fit due to squad assault gun emplacements, grenades, military command structure, air strikes, melancholy weather, disease, and a general lack of treasure. The Italian Renaissance, 17th century France, or the 16th century Caribbean would be much better. If it were Sci-Fi, you would need to explain why guns are not in common use, public misconduct difficult to identify and treasure is carried from place-to-place.
Each session starts after the last ended. Use the downtime to consider the events of the previous session: would a character plan retribution again a player character? Should word of recent events travel and will it affect future plans as many people or groups take action? What is the likely “next step” following what the players did in the last session? If you don’t blatantly plan against them, your players should openly tell you their plans, making it easier to plan the next session.