If you're very new to 3d modeling and want start modeling people right away, you might want to try out Milkshape 3D.
It's a super easy low-poly modeler with built-in animation and uv mapping system with a good number of file type support. It's very cheap too.
Here is the link to the Milkshape 3D website -
http://www.milkshape3d.com
If you decide to use Milkshape, check this tutorial out -
http://www.machinima.com/articles/modeling_intro_index/
It's very easy to understand and it's geared towards people who never have modeled before.
The tutorial page on Milkshape website has many more links.
http://www.swissquake.ch/chumbalum-soft/ms3d/tutorials.html
And for your other questions, cinematic models are usually high-poly, or, very intricate models that are not used for gameplay. Low-poly modeling programs like Milkshape can't create very high poly model, and also can't offer the advanced features needed for cinematic rendering. But more powerful and costly 3d modeling packages like 3D Studio Max can render cinematic sequences yet also create low-poly in-game models. So, it's up to you really, if you use the same program for cinematic rendering or not.
World creation or level designing is a bit more complex question. You can most certainly use the same modeling package to model your level objects or even combine all the objects into a single file, but if you want to add enemies and other dynamic objects in your game, you'll most likely need to make yourself a world editor. As low-poly model editors like Milkshape 3D can't process a great many polygons in a single file, which you game world might need, bigger softwares are better in this case too.
But you should consider things like price and ease-of-use too. The Milkshape demo is full functional for 30 days, check it out, you might like it.
