You’re going to have to defer your project for a little while so you can teach yourself the basics of the program. You are obviously not ready to jump into a project. “The basics” would include an introduction to the capabilities and operation of each of the tools and to the major elements of the program interface. Until you reach that point, it’s almost useless to try to tell you how to do something, because there’s just too much you never heard of.
Don’t worry if the basic tutorials are more geared toward architectural subjects; it’s still all stuff you can use. There’s not really much difference between a ranch house and a breadbox except size. There are only a handful of tools, and once you know how to use them, you can model pretty much anything you can think of.
You can get an idea of the various specific forms of instructional resources there are here: http://www.sketchup.com/learn. Whether you prefer videos or self-paced tutorials or the written word is up to you; all are available.
At the very least, you should try to learn about each of the tools and dialogs. At that point, it may all start to get pretty obvious how to proceed. At the very least, you’ll be able to ask far more focused questions and have some idea of what to do with the answers.
As to the questions you have asked:
What template is best to use? It’s mostly a matter of preference as to what units, fonts, colors, styles and other settings you like to use. The bundled templates are mainly to give you a starting point. You can individually set each one of these preferences even if you use the “wrong” template. Sooner or later every user creates a personal custom template with the settings he uses most often.
How do I make the lines an exact length? You enter exact dimensions into the Measurement box of the Measurement toolbar as you work. You should spend some time reading the material in the Knowledge Base about the Measurement toolbar. You can also reference geometry you are drawing to other objects or the axis system using a technique called “inferencing.” Learn as much about inferencing as you can.
Can I take a model and do an exact mirror copy of it, and if so how? Sure. There are two ways to mirror an object: you can use the Flip along [axis] command or you can scale an object to -1. There are also mirroring plugins.