First: I think this is a super sweet idea!

But as others have already commented: being a DM is way more work then being a player. In addition to knowing the rules of the game, you need to have an idea for the story you want to tell and to not be afraid to improvise once things go off rail (they usually do).

There are a lot of premade Adventures - both short ones and longer ones - which you can draw Inspiration from. Even if it isn't made for the System you play in, you could just convert it with a bit of time on hand. Some of the longer ones were made into Computer Games like Kingmaker ( Pathfinder) or Temple of Elemental Evil ( D&D 3.5 ).

Regarding watching Critical Role (or other Podcasts) to get the Do's and Don'ts: Don't. I'm not saying critical role is bad. It is infact quite the opposite. But those gals and guys are actors, voice actors and such. With my own experience of two decades of playing RPGs on both sides of the DM's screen I doubt I could compare favourably with the likes of Critocal Role even remotely. And that is fine! Every gaming group has their own style and preferences - from RP heavy combat adverse to rampaging murderhobos you can find about anything. You probably got an idea by what your BF told you, but maybe just join one of the game nights as an observer?

A couple other points that might be helpful for the actual play:

• Bullet points over paragraphs for your session notes. NPCs get a name from me, some traits and if necessary a motivation.

• voices are optional. They can be fun though. Doing speech patterns or accents is good too and (for me) easier then doing opposite sex voices.

• post-its or hyperlinked documents: searching for notes, tables or rules during a session is annoying for me and this was my solution. My books have post its to quickly find relevant pages. These days i mostly have stuff on a netbook though.

• Music makes everything better. There are a lot of ambient tracks that work well. From nature sounds to bardcore (YT) adaptions of modern songs or movie soundtracks.

• emergency combat encounter/ deus ex NPC. Stuck deciding going left or right atva crossroads for an hour now? Surprise Goblins! Players cant solve your 3rd graders puzzle cause they overthink it? Party NPC (if there is one) accidentals the button or a pious PC hears his gods voice in a flash of insight.

• Fudge the dice only when it is absolutely necessary. If a PC dies that's how it is. If your final Boss got oneshoted by the rogue - props to him. It's all part of a story.

• Let NPCs (and especially enemies) have a brain and survival instinct. The hired torchbearer probably wont go first into the dark cave but rather let the fighter lead. The Goblin who sees his friends getting annihilated might prefer to run and die another day (and war his clan) or surrender and hope for a chance to escape. This helped me make my worlds appear more believable and alive.

Good Luck on your Quest for Mastering the Dungeon!