>> What engine to use?
depends on the game type.
>> 2D or 3D?
your choice. how much do you know already? text? 2D? 3D? how much more do you want to have to learn? how long do you want it all to take?
>> And should it be an RPG game?
text or graphical adventure would probably be the best choice for a story heavy game (IE where its primarily about the story). a storyline based shooter would be the next step up, followed by a storyline based rpg, and then perhaps a storyline driven simulator / simulation, depending on game setting and game depth. graphical adventures, shooters, RPGs, and sims could be 2D or 3D. shooters, rpgs, and sims, (and maybe the others too!) could be real time or turn based combat.