You've decided to be a Dungeon Master, but you're not really sure what to do. How do you make an exciting game? How do you know what to do next? What if the players want to do something you didn't expect or plan for?
"...and then we begin the invasion..."
These are all valid concerns. But, as DMs, it's not really our job to push a story, drive action, or shepherd the players toward a desired outcome. While you might create schemes and plots for your bad guys, the outcome of those best laid plans really depends on the action (or inaction) of your players. Really most of the game-play revolves around this question "What do you do?" and the players' response to it. And when they've answered that question, when they've told you their goals, that's your opportunity to find or create conflict.
Conflict is the heart of the game. Victories and tragedies spring from conflict's seed, as do the most memorable moments and spectacular stories. Conflict is our true objective as Dungeon Masters.
To create Conflict, all you need to do is this; everything else is variation on it:
1.) Identify your players' goals.
2.) Put an obstacle between them and their goals.
3.) See what happens.
You're going to describe a scenario in the game world & posit an open question. Then the players will attempt to answer that question. Once the question is answered, you move on to the next scenario.
Here it is in Mario: Bowser, the Evil Dragon-Turtle King, has kidnapped your friend and all-around popular nice girl, Peach. Can you rescue her before Peach's father surrenders the entire Mushroom Kingdom to Bowser?
Scenario: Bowser kidnapped Peach, demanding her father surrender the Mushroom Kingdom as ransom.
Goal: Save your friend, save the Mushroom Kingdom, & thwart Bowser.
Open Question: Can you achieve your goal before Peach's father surrenders to Bowser?
Thats an adventure right there! Now as your players discuss how to save the day, you'll note they come up with plans - goals - in order to maximize their chance at success. Note those goals. Put obstacles in the way.
Mario: "Alright, we should convince Peach's father to hold out. Give us time to pull a rescue."
(Goal: Acquire time. Obstacle: King's Fear for Peach's Safety needs to be overcome. Question: Can you convince Peach's father to put the safety of his kingdom ahead of the safety of his daughter?)
Luigi: "We'll have to get into Bowser's Castle to rescue Peach."
(Goal: Rescue mission. Obstacle: Bowser's lair/dungeon, including minions and Bowser himself. Question: Can you infiltrate Bowsers lair and rescue Peach?)
Toad: "We should kill Bowser while we're there."
(Goal: Kill Bowser. Obstacles: Bowser's Lair, Minions, & Bowser - hey we already have this! Question: Can you kill Bowser or will he live to menace you another day?)
Mario: "That's cold-blooded, Toad."
You can keep breaking this down, bit by bit, until you're at the encounter level...
"Can we get past Bowser on this narrow bridge to where Peach is imprisoned?"
...and further down to the action level... "Can you jump over Bowser without getting stopped, hurt, or hurled into lava?"
The opportunity to create conflict exists wherever your players have established a goal or objective. The obstacles you find, place, or invent are the sources of conflict.
All that leaves you is how to resolve actions. And that's easy!
1.) Will this action work? If Yes, just say so. If No, just say so. If maybe, proceed to 2.
2.) Does this action (which might succeed OR fail) have a meaningful consequence? If No, don't bother asking for a check, just let them succeed or fail, based on whatever's appropriate to the circumstance. (I usually let them succeed). If Yes, proceed to 3.
3.) Determine if the action is Easy (Difficulty Class 10), Medium (DC 15), Hard (DC 20), or nearly impossible to accomplish (DC 25).
3a.) Tell them, "That seems [easy, medium, hard, or nearly impossible]."
3b.) Determine what ability check seems appropriate (Strength, Dexterity, Constitution, Intelligence, Wisdom, or Charisma) and say, "You'll have to make a [Difficulty] [Ability] check."
4.) Tell them the consequences for success and for failure. "If you make it, you'll [blah, blah, blah - whatever their goal is here], but if you fail then [take damage, impose a consequence, complicate the situation, whatever]." Then ask your Golden Question, "What do you do?"
5.) Player takes action (or not) and you describe the scenario based on the new outcomes.
This only seems complicated at first. In reality, you're always doing the same thing, over and over. Identify goals, set obstacles, respond to player actions with (You succeed! You fail! Give me an ability check!). Rinse and repeat.
All Dungeon Mastery, action, and adventure - all of good storytelling - depends on Conflict. Find or Create it in your campaigns, adventures, scenarios, encounters, and even in your actions. You'll be fine.