im jus getting into this stuff w my set design studies. heres what ive learnt.
sketchup - very very easy, free, has a lot of pre-made objects you can drop in. comics wise, its good for creating a scene and then tracing it / using it as a reference in your drawing. for realistic rendering its not great, but for getting down a plan for a scene its very good.
blender - free, again, great tutorials on youtube. very easy to pick up, very good rendering - blenders just great. i picked it up in about 6 tutorial videos, it has a lot of great features, like a very accurate water simulator, and you can find more for free online. my personal recommendation.
if youre willing to fork out (or get your hands dirty in the backalleys of the internet,) cinema 4D is also very good for 3D modelling. its expensive as SHIT though so just pray it falls into your lap