Or, for example, instead of making a full RPG, start with the battles. I'm guessing you wanted to make a turn based RPG.
You can pick the mage, the soldier and the archer (for example) and you can pick your opponents (rats, skeletons, or maybe even a dragon) and you fight. The usual: player action, do the math for the HP lost (maybe even use the "miss", and "critical"), then the AI.
And there's where it gets tricky. A good AI can do wonders for your game, whereas a bad AI will probably kill the whole project. Will the dragon attack? Will he use his turn to recover MP and so the next turn he can use his "Breath fire" spell? Will he kill the mage, or maybe cripple (cripple as a status effect, where maybe he can only land half of his hits) the soldier if he's the main character.
And honestly, there are tons of things you can add later. To be honest, while I'm writing this I'm imagining how MY RPG would be like. I love advance wars and fire emblem series, tactical turn based RPG me likes.
To conclude, don't be frightened because of a big project, divide it in sizable chunks and then eat them one by one.