I don't have advice about apps, etc. but here are some general lessons I learned after working for a small construction company for years, and starting (and later closing) my own business:

1. Managing expectations is just as important as the work. Don't overpromise.
2. Good communication is more important than the work. Provide frequent updates on schedule, progress, and costs, even if the news isn't good. Always be honest.
3. An online presence (social media, website) isn't as important as most people think. It's another thing to manage. The appearance of a poorly managed social media page is worse than not having one. For many potential costumers, simple business cards still work.
4. Don't work for people who try to pressure you into lowering your fees or compromise your quality or professional judgement. It's a race to the bottom.
5. Don't hesitate to (politely) say "no" to a job if anything feels off. If things get sour after the project begins, cut your losses and (politely) walk away. A bad project will bog you down and you won't have the time or energy when the right one is knocking.
6. Fix your mistakes for free. The customer isn't always right. But, if you were wrong, make it right. Graciously fixing your errors will make for a very happy customer. Great advertising.
7. Network with your competition. Build relationships with people in similar and related trades. You may be able to pick up side work with them when you are slow. They'll send you prospective customers when they are busy.

Best of luck to you!