Which piping 3D CAD software should I learn?
Blender3DOpen Source project developed and maintained by community.Has lots of tutorials to support learning due to large community base.Has features of the entire pipeline in one software.ModelingSculptingRigging and AnimationTexturingLighting and CompositingVideo editingVFXGame EngineWhat are you waiting for. The best of all, it's free!Download here blender.org
Which cad software should I learn for modeling?
That depends on what you're modelling and for whom.SolidWorks is the answer that first comes to mind-well because I've used it for so long-but it's mainstream CAD designed for machinery, electro-mechanical appliances, consumer products. It has the broadest reach and if you're planning to invent something, do design as a service or work for a US manufacturer making anything that's not vehicles with thousands of unique parts, then chances are, SolidWorks is the best value for the moneyIf you're getting into large scale manufacturing, particularly defense, medical or power generation and particularly if you're looking to massively configure products and automate product definition, the Creo is your best bet. It's more expensive than Solidworks and has fewer installed seats so this is software to learn if you know the place you're going is using it.If you need advanced surfacing capabilities, large scale integration, like for power-plants, aircraft, research and development then Seimens Nx is that next level platform.Finally, if you're Airbus, Pratt & Whitney, Chrysler, Volkswagen, Jaguar or one of these companies that can't refill the tampon dispensor in the bathroom without having a digital mock-up, simulation and certified supplier, then your first choice is probably Catia V6.On the side, whatever, you have at hand or know how to use, go with that. It's much more important that you learn to express and develop your ideas than it is for you to use a specific CAD platform.
What is the best software to learn for 3D modelling and CAD? And where can I start to learn it?
That's a very broad range and there is no one software that really good for all of them.3D ModelingCharacters: Not my field but Poser is relatively easy and I gather that Modo is quite good.Environments: This one’s hard. I'm not sure what's available for games and the stuff I know is very technical for civil engineering and very expensive and complex.Objects: Lots of options here. The easiest is probably SketchUp but it's ability to model curved surfaces is quite limited. Rhino is probably the best all around choice for this.Special effects: Not my field. I'm passing on this one.Products: Fusion by Autodesk or OnShape are good low cost options here. Solidworks is amazing but very technical and very expensive.Robots: This is tricky as you need a combination of object/product design and rigging for animation. Also not my field but I expect this may require a couple of different programs for best results.Architecture: SketchUp is likely your best choice here as it's capable, easy, and free (for non-professional use). ArchiCAD is IMO the best for design and Revit is good for engineering but they are both expensive and complex professional software.Etc:Landscaping is tough. There's good stuff available but high quantity or quality will crush your hardware.Vehicles are tricky. Your best bet with them is to use or at least start with ready made assets.CADIf you mean Computer Aided Design, see above.If you mean Computer Aided Drafting there are quite a few programs available from free to quite expensive. I am guessing you don't really need to do technical drawings so I assume this is not what you mean.
Which software should I learn? Solidworks, Catia, or Autocad (Autodesk)?
First, I'd likely stay away from Catia. It's great software but from a more practical standpoint you are more likely to find employment with a company that uses Solidworks than a company that utilizes Catia. Next... you can't go wrong with learning AutoCAD. At some point in your career I can practically guarantee you'll use it at least to an extent. Solidworks vs Inventor... tough choice if we're talking long term, both have a bright future and really you can't go wrong learning either one. The end answer to your question: I'd learn AutoCAD as well as either Solidworks or Inventor. You'll find that more and more individuals are using multiple software products to achieve their designs and the more programs you are familiar with the better deliverable you'll be able to produce (and the more marketable you'll be in your career).