Hi there,
When you're new to 3D modelling I would suggest to make everything by yourself and refrain from using pre-made objects. The reason is that you will need to get a proper understanding of a 3D workflow before you start using other people's models, and you still need to work on your modelling experience.
3D isn't something you can master in a week or month. It can take a few months to grasp the basics and establish a workflow, and years to refine it.
So if you need custom buildings, there is only one thing I can advise: Make them yourself. Search for photo's to use as reference materials, go to www.textures.com for proper textures, and start modelling. Even if you've only progressed a little and learned one thing, you still progressed which is ever so important.
Also, search for some tutorials. They'll explain you the ropes of the software you are using.
