Did you have a mentor when building your first startup and how was it?

Yup, helped a lot. Had a ton of ideas. The mentors helped refine them. When you're in creative mode, you're not always in rational mode. I'd bounce ideas off the mentor(s) every day, which also forced me to build something to show them.

Avoid mentors who are regular cynical corporate job types though. They do more harm than help, and I think they subconsciously just wanted us to see us fail. Find the ones who are cheering for you.

Had a partner, with extensive experience and ready assurances that all I needed to do was the programming (and packaging and distribution and sales and support...) and they'd take care of the rest.

That one cleared about $500k before dying; I came out of it with debt and not enough knowledge or cynicism to avoid doing the same stupid shit again.

Build relationships with paying customers ASAP in the journey. They will give you all the guidance you need.