I'm such a terrible builder, I have mostly just used the premade houses in the worlds. I've occasionally tinkered with them (usually adding basements and/or sheds), but that's about it. Practice makes perfect, and I'm currently working on making a decent ranch out of the big empty lot in Chestnut Ridge.
For businesses, I have downloaded most of them from the gallery, and I always fill Newcrest up with them as to be a business district for my Sims to shop at. My first designed-by-me business was when I gutted ThrifTea and made it into a hardware/farm and feed for Horse Ranch, as it appears ThrifTea, to begin with, was a repurposed general store.