My #1 advice is to pick an idea you enjoy working on.
Market size and revenue potential are important, but if you don't work on something you truly enjoy, motivation will vanish within a few months.
And since startups can take years to get off the ground, you'll quit before reaching the finish line.
Whereas if you pick an idea you enjoy, you'll be able to keep going through the ups and down. Until eventually you succeed.

Bonus point if it's something you enjoy working on AND it's for customers you deeply care about 🔥

My wife has been a logo designer for 15 years. And she regularly had startup founders come up to her to design their logos. But since they were just getting started, they couldn't afford the $3k+ price for custom work.
So we thought of a way to automate her work and design process in order to lower the price. That way even new startups can afford it.

Right now it's 99% through twitter. I tried a lot of different acquisition channels (google ads, newsletter sponsorships, online communities) but none of them really worked. Then I started tweeting and realized it was the perfect medium to connect with other founders and build a relationship with them. And since they all need a logo at some point... eventually they remember our startup and become customers.

We got our first ones by announcing the launch of our startup on indiehackers, reddit (r/startup) and a few other online communities. It got us our first 10 sales as people were curious to try it out.