From time to time, I've shared the work of one of my model railroad friends, Ray, with you. He is a master scratch-builder as demonstrated by these models of the Wabash station in New Paris, Indiana. It is rumored that Ray has assembled a kit or two in his life but he is much more comfortable with a few sheets of styrene roofing & siding, a handful of Tichy windows & doors and some plastic cement.
Ray's work is remarkable in that much of what he does is of buildings and other structures that no longer exist. He will see a picture of a station that he likes and begin his reasarch. When he finds a sufficent number of photos, he then picks a feature such as a door or window that he is reasonably sure of the width or height. This gives him a reference point and from there, he will make up a sketch with dimensions of what the building will look like. He lays it out, cuts it out, assembles it then paints it and he has a very good model to show for his efforts.
Perhaps you feel a little intimidated by his work but take a look at the sheds in the background of this photo. Ray built these as well. In many cases, they are a simple four walls, sloped roof and a door or window. Some are no more than a lean-to attached to the side of another structure. They would be very simple "first try" projects that would allow you to learn how to measure, cut & assemble styrene. I know from first-hand experience that you simply dive in and have at it. Some things you can only learn by immersion. Tool needs are simple as well... all you really need is a good steel-straight-edge, a good Xacto knife with several sharp #11 blades, a good ruler and a few files or emery boards to clean things up a bit. Oh yeah, if you are a bit clumsyclutsy like me, a band-aid or two might be handy as well. Those knife blades are sharp. :)
Even if you aren't motivated by these photos to go out and grab some raw material and start your new building career, you can appreciate the work Ray has done here.
Keep in mind that many of us model that time about 55-65 years ago when there were small structures/sheds everywhere. You had maintenance sheds, lineside phone booths, handcar sheds, small bunk houses, storage sheds, simple passenger shelters, outhouses and a myriad of other simple buildings that would make great first time projects. So don't be afraid to simply have at it. Styrene shapes & sheets can be found at most any hobby shops (Evergreen Styrene is really good to work with: http://www.evergreenscalemodels.com/) and Tichy windows and doors can be ordered directly from Tichy Products. Their web site is: http://www.tichytraingroup.com/index.php
Above all else, like the Nike commercial says, "Just do it!"