You'll find lots of posts on the forum that list the lessons learnt that each of us has experienced when creating their smart home. I totally agree with @csteele about starting small and simple and then expand as your feel more confident but IMHO with one word of caution: I made the mistake of buying some devices because they were discounted at the time and so ended up with a myriad of devices from different manufactures.
This isn't always an issue but keeping track can be harder down the line. You may want to look at my post here which explains my approach to device's per usage and documentation approach (Sorry if some people consider that I swore then!) 
.
