Renderings, 3D CAD models or Photo manipulation?
Short answer:
Yes, there is software available that can find/download to get you started in either of these. I'm sure someone can jump in with some links.
Adobe Photoshop is a must and Adobe Illustrator is also great for renderings. There's more fundamental software that's less expensive or even free but they're extremely limited. Photoshop is the industry standard.
As far as 3D CAD there's a bunch of options with a variety of availability, capabilities and price. The most powerful ones are obviously going to be more expensive. Alias, SolidWorks, 3DSMax, Rhino, Maya, Vellum, Vector Works, Modo are some and I'm sure there are more. I used Alias for years but at multi-thousand dollar prices for even a basic student seat, it becomes cost prohibitive. There's bootlegged stuff out there I'm sure.
Long answer:
I'd start by drawing the "old fashion" way...pencils, pen, charcoal, pastels, paints, ketchup, whipped cream....whatever. There is no quick way to do a rendering/illustration of any type unless you're just altering photos. Anyone with access to Photoshop and some practice can do that.
Using software is just a tool to convey what's trapped in your head, it's not going to turn a novice artist into Michelangelo.
When it comes to 3D CAD, that's completely different. There's no fast and easy way to build an accurate model of most complex objects like cars, unless you're purchasing an existing model and tweaking it.
If you're looking to do "renderings", get some how to draw books first, then spend time drawing and understanding the way objects look in space. Understanding perspective is key to a good rendering. All the Photoshop effects in the world and a Wacom tablet won't hide a badly drawn perspective.
After a lifetime of drawing cars, I still pick my own stuff apart and push myself to do better on the next one. Never stop learning. I started using Photoshop in 99 and I'm fairly proficient at it, yet there's so much I don't know.
Good luck and be patient.