If you've never written any games, I recommend you start with a simpler game idea, like Pong, Breakout, Tetris, etc. so you can get a grasp of what some a project can involve.
From there you should start scratching out, what you actually want. 2.5D RPG can be alot of different things. Make sure you know exactly what you want with the 2.5D stuff.
With written down idea, it's easier to layout a plan and start to split things up in different tasks, which you can check of, thus enabling you to see and nearly more important feel the progress and be able to deal with the loads and loads of work.
Just starting coding and doing stuff will quickly put you into frustration and leave behind quite a mess. But sometimes, with enough pation, dedication and motivation , doable as well.
In both cases, one of the most important things to keep in mind is: Don't fear the refactoring!
