Godot 4,5
What are some factors should i consider? Im an amateur
in this so like im not that good but i got the basics in and are there any good tutorials or do i have to figure out myself? If so what are things i should focus on learning more abt or does general game dev knowledge in godot alr make it possible?
“Factors” is a bit too generic to get a good answer. The simple answer is “everything” because that is what you are responsible for when you create a game.
To get a bit more specific, what is it you like about the games you mentioned? The story? The mechanics? The genre?
What do you want your game to be? What do you want it to do? How will it be similar? How will it be different?
Answering these questions will point you towards what you need to learn, which will in turn point you to the tutorials that have the information you need.
If i remember right, the undertale dev used fairly basic coding to make the game. The game requires a lot of 'flags’ to be remembered in terms of what you did and also a way to logically group and store dialogue. Even more than coding, you will need a good way to visualize all the branches so you might want to look into programs that help you organize your narratives and dialogues.
It’s a very ambitious project to start off with! I’d suggest focusing on the mode you’re most interested in first, like the world map or the fight mode, and seeing where that gets you… in Godot, you can easily create different scenes for different modes, and link them together later.
As for plugins, I recommend Godot Dialogue Manager (by Nathan Hoad) - it does talking head style dialogue very easily, and lets you run code from dialogue, among other things. You can install it from the Godot AssetLib, where you can also find a video tutorial on how to set it up.
There may be a suitable plugin for building the world map as well, but I’ve never used one 
Small world, huh?
To teach myself Godot, I’m also working on a Deltarune fangame, and I’ve spent literally a month (I started the project on the 31st of March) working on perfecting how THE DIALOGUE BOXES look and feel.
So yeah, maybe not the best idea for a first/easy project.