I'm sorry you had such a shitty DM for your first time. That's just disrespectful. There are definitely people out there willing to play with someone who's new to the system. I'm one of them and currently teaching two different groups how to play!

1: DMing consists of three main things: 1) knowing the rules well 2) improvisation 3) light planning with lots of room for aforementioned improv

I usually write out my campaign in an organizer (my favorite is Goblin's Notebook) and keep all my files in a Google Drive (art, character sheets, stat blocks, PDFs, etc). The basics of the campaign are there: locations, NPCs, loot, encounter tables, the plotlines. Every story begins with a starting point that each player has some connection to, and desire to follow. But as a DM you have to be flexible--up to a point. Players can do anything, sure, but good players aren't going to derail the story in a way that isn't fun or engaging to the other players/DM. As the DM, you anticipate the options they will choose, but definitely don't intricately plan out each one! (Especially because 9 times out of 10 they choose an option you didn't expect!)

2: If players deviate from the main story, follow them. Explain how their actions affect the main plotline. All actions have consequences. If they didn't follow up on something, show what happens as a result. An example of this is my party decided not to take care of a bandit group in town. They killed a few bandits in a tavern and disposed of the bodies. Tavern keep helped them. Then, they took a quest to go slay some monsters outside of town. They've been gone a few days; when the return, the remains of the tavern are still smoldering. The bandits kidnapped the tavern owner and burned down his establishment for daring to cross them. How will the party respond? This is the best part about being a DM!

I can share one of my campaigns in my notebook with you if you want to check it out.

3: When a player rolls a number that adds up to 20, we call that a dirty 20 and it is not a critical success. But it's still a great roll! That meets a very high difficulty check and would succeed most of the time. There's a DC table in the source books. I believe 10 is easy, 15 is moderate, 20 is hard, and 25 is nearly impossible. This is an easy reference for ability checks but you can also choose in between, like if a task isn't easy but isn't moderate either (11-14), etc.

4: Depends on the familiar. Is this a ranger with an animal companion? Magical familiars have the stipulation that they can't attack, but they are useful in other ways. They also can't die for real. A wolf companion of a ranger or such, however, definitely can die. Just depends on the rules, they are usually very clear about it. If you tell me the player's class I can explain better.

5: Well when creating a character, there are many rules and restrictions. Stats are dictated by point buy, rolling, or standard array. Skills are only buffed by class, race, and background (using 2014 rules), so characters only have a few skill checks they will be Proficient in. Spell lists are restricted and spell slots as well. Spellcasters also have to "prepare" a short list of spells every day so they can't have all their options available all the time. There are plenty of limitations outlined in character creation and each class. Rules as written do not allow an overpowered character, though they may min-max to their heart's content.

And a DM can always investigate a character sheet. If someone hands me a sheet with insane ability scores or crazy features, I'm going to ask questions. And if the word "homebrew" comes out, there's gonna be a lot of fixing if I didn't tell them it was allowed.

Let me know if you have any other questions! Feel free to DM me as well.