What is your goal? Do you want to make a business, learn something or just for fun?
As Florian and Marc mentioned it's a good idea to solve a problem. I can also add that making something that you're not using yourself is more difficult as a one man team, unless you really like talking to users and performing user tests, in addition to making the thing.
Yes! I think it's one of the most important parts. Users trust you with their data. I don't use any tool but I write tests for critical parts of the applications :). There's also a lot of good practices to make programs more secure. I use OWASP Top 10 as one of my references, you can find it here: www.owasp.org/index.php/Top_1…
I've used Clearbit before and it worked really nice if you don't want to bother users or worry that a challenge like CAPTCHA would impact signups too much. You can check out the Risk API, they have a free plan for low volume clearbit.com/risk
A beautiful and easy to use offline maps app 🙃
Try maps.me, works perfect. Bases on the map data of Openstreetmap.
I use goodreads to search / log all my books. Not much experience with promotional pages but things I look for (non fiction):
Then I'd put it on a to-read list and come back to it later.
Hope it helps!