Click on the "Resources" tab, then click on "Structure plans". Then click on the desired structure. You will then get a group of sketches with directions on how to build that structure, including printable PDF's of plans (in N scale). I would suggest you start with a simple one like this https://www.trainboard.com/highball/index.php?resources/small-signal-shed.78/ Just follow the directions. Note: These require that you cut out wall sections from styrene or wood. If you can't or won't do that, then you need to build from kits or get ready made.
That's a good link. I'm always interested in building stuff from scratch. Time is the big issue. But it is still a very fascinating part of the hobby. Best "old school" book I've seen on scratchbuilding is Wayne and Mary Wesolowski's "Model Railroad Scratchbuilding". Was at a train show in November and picked up a copy for $7.50. I've also seen them on ebay. Very good reading. If one has any skills at all, this is an excellent book. It was published several decades ago, so it is very text heavy w/ some photo's and diagrams. The reading is great and it really explains that scratch building is nothing more than diving in, taking your time, and coming out w/ something you'll enjoy building. Since I purchased mine, I've seen it on ebay a couple of times. Thanks for the link.
The Wesolowski's books are an excellent read and a treatise on how to scratchbuild. I managed to get all of their books while in the US, either new, or second hand through library sales and eBay. Don't regret those purchases at all.
Another good source for info on styrene scratch building is Jack Burgess's article he wrote in RMC years back. Found it on the web tonight.. http://www.yosemitevalleyrr.com/about/styreneconst.pdf