http://www.blender.org
Download that, grab some tutorials, and settle in for the long haul. Learning to create a 3D model requires the same amount of time and dedication as any other art form, be it painting, drawing or sculpting - you can't expect to jump right in and create professional-level work in a week.
As for acquiring game models: that's not something the 405th is generally happy to discuss. Suffice it to say that Microsoft and 343i are aware of our using their assets, and are willing to turn a blind eye as to how we acquire them as long as we uphold the spirit of their T&Cs, if not exactly the letter. If your question is about how to go about extracting game models, I'd advise you to seek answers elsewhere.
I'm sorry I can't be of further use to you, or broaden my explanation further - literally, your responses are 'practise' and 'research', and an awful lot of them.