Blender is a free open-source 3D modeling and rendering tool. For more information visit Blender.org
In age building, Blender is currently the modeling tool the GoW recommends for newcomers.
Download the correct Blender version for your operating-system. For Windows i'd suggest the installer rather than the zip version.
Install Blender. This varies from operating-system to operating-system but it should be pretty easy.
Getting Started with Blender
There's a ton of tutorials there, but for basic age-building it's extremely important you cover the material in the Noob to Pro tutorial from "Unit 1" to "Penguins from spheres". Without being exposed to these tutorials (or tutorials that cover similar stuff) making ages in Blender will be very frustrating. It's very hard to pick up on how to use Blender by just playing with the interface. Building ages is a complex thing to do. Setting some time aside to read those tutorials will help make the process more easier and rewarding.
After you get to "Penguins from spheres" you haven't covered everything, but you've probably learned enough to model some pretty neat stuff for your age(s). After making some stuff for your age you might want to go back to Noob to Pro and do a few more of the tutorials so you can do even more stuff.
There are also Video tutorials you can follow as well.