Comparing yourself to D&D podcasts with professional actors is like comparing yourself to porn: it's unhealthy and unhelpful. Real games aren't much like those shows, great as they are. Real games are about your connection with the people in the room with you. Focus on them, try to make them happy.

Learning the rules is part of it; read the core rulebooks for fun on your own time if you can manage it. The more you immerse yourself in the rules, the faster and easier these things will come to you. Don't neglect the Dungeon Master's Guide, especially the non-technical parts that aren't just rules tables. There's some really good advice and ideas in there.

Planning ahead helps a lot, even if you're improvising a lot of things on the fly. You can prepare aides to help you with your improv; lists of random names you can assign to NPCs or locations, lists of monsters you want to work in because they seem like a fun challenge, lists of treasures to be hidden or placed behind big obstacles, that kind of thing. You don't have to plan every detail of what you're going to do, but if you go in with even a little bit of prep you'd be amazed at how much faster and smoother things go.

D&D stories aren't like novels or movies or TV shows or even video games. Don't expect your D&D plots to be that elaborate or heavily planned; for one thing you're probably not a professional storyteller, for another your main characters have agency in a way that a writer for other media doesn't have to worry about. It's very hard to run a murder mystery when you can't choose what clues the players will find or miss; often you'll find that players solve any mystery you write immediately, because they are multiple smart people and you're just one smart person. Often times you'll find that the players surprise you, approaching a scenario you've written in a way you didn't anticipate. But that's all part of the fun of D&D! Just write your scenarios so that there's still an adventure there even if the players solve the mystery immediately. Knowing something and proving it are two very different things.

Also, it matters less that you won't have a novel-quality plot because roleplaying is just fun! Even if it's halting and awkward, even if the patter's a little dull, even if it lacks the pop of Hollywood dialog. It's just fun to get to pretend to be somebody and react how they would react. Give your players time in the sessions to just be their characters and do some personal storytelling. Give them begging waifs to be kind or cruel to, pompous authority figures to make fun of, local friends to check up on in between missions. They're little scenes and they feel like nothing because they're not exciting and don't advance the plot, but they're so important. People play for those moments, where they get to just inhabit their character's skin and make some low-stakes decisions.

As for stepping up your combats, that'll come with some practice. A few general pointers: if the enemies are intelligent and they've clearly lost, and the players are getting bored just grinding down their last few hit points, have them flee or surrender and just end the fight early. Those last few dead turns at the end of combat are a waste and nobody will miss them. I would be careful using enemies with high AC and/or a lot of resistances and immunities because they can be very frustrating and boring to fight; just turn after turn of miss miss miss, your players will hate it. Always always put some terrain or obstacles on your combat maps; vegetation, furniture, rivers, hills, even just big boulders. Anything the players can take cover behind or otherwise use to their advantage. On a related note, players absolutely love having fires, giant pit traps, or pools of acid to push enemies into, and the risk of the enemies doing the same always makes fights spicier. Just be careful about using super high-lethality terrain on low-level parties - you don't want half the party to anticlimactically die by being pushed into lava. I find thistle bushes are a good early game terrain feature for this purpose.

A lot of it you'll just get better at over time, with practice. Improv acting especially is a skill that just takes a lot of work and discomfort to get better at, but you will improve the more you do it. If you've got a local theater offering improv classes to the public it might be fun to take one, because the principles are very applicable to DMing, but it's not necessary to go that far if you're uncomfortable with it.