3 key lessons from my experience advising "tech" companies in Nigeria over the last few years: a threaded series.
1a) before building a "tech" company, build a "regular" company. Then enable it with tech. If your "regular" company doesn't provide a good or service that people want, or you can't do it better than others, your whole business might be Dead On Arrival.
1b) So if you want to start an online fashion retailer, first figure out how to source for the best items at a price customers want. Then ensure you can work out the logistics of handling, transporting, delivering, handling returns, etc. Even before building a website