Long response incoming because as someone who made a lot of mistakes and had a lot to learn, I have a lot to say about these questions.
I stayed in that "new creator" phase for way too long and it caused me to scrap my game and start from scratch multiple times, and I only just now feel like I'm halfway competent at this and leaving that beginner phase behind. So from experience, I would say your #1 biggest mistake can be going in with no planning or very little planning. I'm not saying you have to have every little thing figured out before starting, but it's good to think about the important stuff. The main things I'd consider important being:
- Plot details, even if just a loose plot to start with, having that helps a lot more than just winging it like I did for a while. That just results in the game feeling like a mess and fixing it later isn't easy.
- Your map, as in like the regions town map. Sure it sounds like something that can wait for later, but trust me when I say having your map already designed (even if just a rough sketch) will help immensely when creating your maps and figuring out how to design them.
Another common mistake is a lack of testing. Don't just assume something works because it's simple or you copied an event in Essentials because somehow, it might not work as intended.
As for things to avoid:
- Overloading yourself. It's tempting to throw every cool resource you find into your game and having as many mechanics as you can, but then you have to try to manage it all and make it all fit in your game (otherwise it just comes across as you're using more mechanics to make your game look good imo).
- Building off the last thing, as a beginner, you should definitely avoid trying to do more than you're capable of. Making something small and simple is a good way to learn and practice. Or you can do as I do, where you have one big main project, but you also work on other smaller side projects and you can take the knowledge or skill you learned from that to make your main project as good as it can be.
- Boring map designs. Making everything too straight and square leads to a meh map design. Looking at some official Pokémon maps or taking a look at this and this can help give some ideas on how best to make a good map.
- Some personal issues, but avoid having bad grammar or tons of spelling mistakes. I don't think that'll apply to you specifically, I'm just stating it here for people in general reading this.
I only have a few tips for beginners. Don't ignore NPC's. Sure they seem largely unimportant but when I compare my project now compared to when I first started, everything feels so dull and lifeless because the NPCs just said whatever one sentence line I could think of. Not saying you have to give them all a unique personality with multiple lines of important dialogue or anything, but just keep in mind how NPCs can really affect an area.
Secondly, I recommend taking some time to learn some of the more important parts of game creation. Before diving in, take some time to learn how events work, learn what PBS files are and how to set up encounters and trainers. Figuring out the important stuff will get you in gear a whole lot quicker than trying to figure it out as you go. Looking at events within default Essentials maps and looking at the
wiki are a great way to get the ball rolling, and once you start diving in, you can see how working with Essentials is so much simpler than it first appears. More importantly, if something confuses you or you can't find the answers you need on the wiki, never be afraid to ask questions. After all, learning more is the most important thing you can do when it comes to game creation. Relic Castle is full of people who might be able to help (I'm always down to try and help if you shoot a DM my way).
And finally, and most importantly, going back to what I said earlier, but test, test, TEST. Try what you just made and ask yourself, "Does this work as intended?", "Is there anything weird that I can fix up?", "Is there anyway that I can make this event look or feel better?". You don't have to look for perfection, but if you can see anyway to make your events or your game look or feel better, if it's within your skill range then it's worth doing. Sure, that stage of development can be boring and tedious, but it's so important for the player who will eventually try your game (I'm on my fifth-almost sixth- week of polishing my game before demo release and I'm in pain, try not to leave the entire polishing section of development at the end

)