kind of honest pro tip for becoming a better dungeon master: run a one-on-one campaign for a good friend or your partner or your sibling or something.
how to prepare a lot of content very quickly - having only one player means there isn't a ton of interparty chatter or debating how to storm the castle, so content goes by faster.
how to cater content to fit your PCs - with only one PC, they're essentially a Main Character, and it feels weird to send them through content that doesn't tie into their goals or backstory.
how to balance encounters - it is so unbelievably easy to kill a single PC so you get the hang of action economy real quick, especially if you start at a low level.
how to play and run lots of NPCs - not only do you run side characters and monsters, but you may also play a few complex NPCs to help simulate an adventuring party.
how to run combat quickly - again if you are both the monsters AND most of an adventuring party, you have to go fast otherwise it's just you playing against yourself for like 30 minutes at a time.
how to delegate tasks - giving your player some NPC stat blocks to run for you in combat, asking your player for details about the world, working together to decide what "shared" NPCs would do.
how to roleplay real good - you're the ONLY person your player has to interact with, so you'll get a lot of practice driving the plot through NPCs and setting details in a way they'll latch on to.
and if you want to go to DM boot camp like I did, find a player who:
loves combat but also wants to play a fancy noble who avoids combat by rolling Persuasion when at all possible - you will get very good at coming up with non-traditional encounters like stage play combat, fights with heavy social elements, and how to move them into situations where talking just isn't going to help.
loves all monsters, is tired of human-centric stories, and hates the trope of "this entire species is evil therefore you are justified in killing them on sight" - you get really good at quickly modifying modules and encounters to have less humans and way more complex depictions of monsters and monster races.
thinks you're cool and likes watching you create stuff - ok this one is just nice, it's very heartwarming when you apologize for taking so long for your NPCs to take their turns and your player is like "no I love it!! I'm just watching the drama like (⚆ᗝ⚆)"
anyway thank you for attending my Ted Talk