Here's an interested pair of eyes.
Say a very very simple RPG. What do I need to be concerned with at first?
Again, it depends. How story-heavy do you want your game to be? If there will be a big story, you need to think of a plot, key characters, events, etc. Even if it's not a huge part, the characters at least should be considered. Next, how do you want to organize it? Will you have a Monster class that you can expand into many subclasses? Will you have a Hero class that you can expand for many types of characters (Mage, Fighter, Cleric, or even futuristic like Sniper, Spy, Engineer)? What will each Monster have? What will each Hero have? How will users interact with the game? What level of graphics will you use?
In my experience, RPGs are some of the hardest games to program. The simplest? Fighting games. Try making a simple fighter game, where it's you (the user) vs. some monster (the computer). You can attack, defend, cast spells, use items...but you don't have to worry about plots or anything past very basic user interface.
I'm planning on entering the field of programming video games, so I'm in no way trying to daunt you...but from my own experience, making homebrew versions of popular games is tough on its own - creating your own game from scratch will be a challenge. Make sure you understand that before diving headlong into the hours of coding. Again, I'm not trying to dissuade you - by all means, dive away!