It's cool that you haven't STed before in a tabletop setting. Everyone's got to start somewhere.
First off, run a game YOU want to run. You said you already have a game/story in mind. That's great. Write a good description and boldly state "new players welcome!" For some reason, people will only join a new game if those magic words are uttered. I don't know why. Also include any parameters/restrictions you have for the game: no Tremere, no Moros mages, 70 freebie points for all, etc. In case anyone wants to fill out their sheets ahead of time.
I recommend that you have some kind of system in place that explains why the characters would be adventuring together. Players want to play whatever characters they have in mind and won't fit neatly into whatever story you're writing. So you'll have a gunslinger, a rabbi pacifist, a workaholic banker, and a sexy cat boy as your PCs with contradictory goals. As an ST, their constant bickering will burn you and your story from both ends. I recommend you lay down some ground rules. The longest-running, successful games had some kind of unifying code or pact built into the story: a team of detectives, an elite group of mercenaries, one side of a war, a cult...whatever.
One more thing about inter-character relationships. The Dresden Files game I ran, imo, had a great solution to "why would these random PCs be adventuring together?" by forcing other characters into each other's backstory AT CHARACTER CREATION (the first session). We used this for Shadowrun with great success. It unifies the group and fleshes out people's backstories. One player wants to play Princess Cashmere, a steampunk heiress. Another player wants to pay a ninja assassin who is hiding a Dark Secret. The ST made these two create a backstory...story... together during character creation. The players came up with the Ninja social-engineering his way to stay with her parents while he was on the run. Her parents liked this charming man and now try to set her up with him at every chance. Setting up these kinds of relationships between characters allows the players to interact with each other, in addition to interacting with the ST. The best sessions are when "the game runs itself" and oftentimes it's because the players interact with one another. It's easiest to set this up from the get-go.
All this would be done during the first session, which has basically been half character creation and a short, simple adventure. While some players will come with their sheet filled out, you'll need to, at the very least, learn character creation. If you don't feel like reading the entire rule book, that's fine...but you DO need to read about character creation.
All that advice was for setting up the game. As for actually running, well, everyone's got their own ST style and there isn't a right or wrong way. Some STs (me included) run anything-goes free-for-alls while other STs highly-structured, rules-heavy, complicated dramas. The former STs say yes to everything and don't have linear plots, which infuriates structured/rules-heavy players who are then left unsure what to do. The latter STs make you roll dice more, police your stats more, (and like battles more, for some reason) which infuriates players who want to roam free and raise hell. Maybe you already know what kind of ST you are.
Biggest ST problems:
haha, didn't plan on writing you a novel here. Hope I covered what you wanted me to! If not, feel free to ask me to elaborate on anything!
- Overplanning: I can think of a certain game where an ST had everything detailed out, spent hours and hours planning, and was very disappointed when he had to throw out everything in the first 5 minutes. One of the STs in the group has a saying to combat these inevitable derailments: "All roads lead to Rome." Just start steering the story back to where you want to go. Be VERY careful this isn't too heavy-handed.
- Somewhat related...a paralyzed ST: I don't know what causes it, but sometimes the ST gets too possessive of a NPC or plot point, or there's a PC who's driving you insane and you just freeze up. Fights over rules. These are worst case scenarios that you fear. The best solution is "say yes or roll dice." An overpowered mage who always ruins your story wants to cast a fireball that blows up the mansion you spent hours stating out? Say yes to the player or make them roll dice. Put a difficulty. Justify it. If you don't agree with the rules in the book or want to make this a "special case" it's perfectly ok to make up your own roles. You'll see how your table reacts to this...It's ok to poll the table and ask, "does this sound fair?" most PCs side with the ST in the interest of moving the story along.
- Running out of ideas: probably your other biggest fear, right? when everything's derailed in the first 5 minutes and all your planning is out. It's perfectly fine to stop the game and go off alone and collect your thoughts. I one time did the worst thing ever. I said "well, you killed my boss in the first hour. that's all I got. see you next week." Uh oh. People weren't happy. I ended up stepping aside and reworking stuff that wasn't used that session and added some antagonists.
- Not knowing ALL of the rules: Don't worry about this. Your players will help you. My experience is that the best STs don't necessarily have eidetic memory of the rule books. We've run games new out of the box and learned along with the ST.