Need Advice: Validating My First SaaS Idea

I just started building my first SaaS, and honestly, I have a lot of questions.

I have an idea that solves a problem I personally face, but I'm not sure if enough people would actually pay for it.

A few questions for founders who have been through this:

How did you validate your SaaS idea before spending months building it?
Are there any tools or platforms you recommend for idea validation?
What's the biggest mistake first-time SaaS founders make?
If you were starting from scratch today, what would your marketing strategy be before launching?

I'm trying to avoid building something nobody wants, so I'd really appreciate any advice or lessons from your experience.

Thanks!