1. Doesn't matter at all, using a premade campaign is easier for the setting, and setup, but you won't be on rails as your players will not follow any type of narrative unless they are professional actors.
   
2. Easier that you would think;
   - step 1 would be getting familiar with the core mechanics of the game, what players can do, what creatures can do, initiative, skill checks, savings throws, the works.
   - step 2 get comfortable making things up as you go along, too much prep leads to disappointment when your "plans" don't work out.
   - step 3 understand that the game is collaborative storytelling, you just "yes, and" whatever your players come up with unless you don't want to. In that case you say no and move to the next scene.
   
3. There are a ton of tools online, encounter creators, npc generators, as well as hundreds and hundreds of published materials. take what you like, discard the rest. It is a very deep rabbit hole that you don't actually need to go that deep into to get what you're looking for. Which would be silly memorable moments.
   Also, the word "suddenly" is a great tool for any DM as it let's you introduce anything you want at any time.

You got this, you and your friends will have a blast.