You can read about the general process on the Secretary of State's website. The specific criteria is informed by constitutional provisions (ctrl/cmd + F and search for initiatives) and state law.

Anecdotally I can tell you it's really ^#$%ing hard. For instance, the first obvious and raw hurdle is getting at least 8% of the number of votes cast in the last gubernatorial election. After 2020, that number is about 325,000 valid signatures. You typically need a buffer to account of signatures that Secretary of State's office invalidates for various reasons. So let's round that number up to 400,000 or more. Let's say you give yourself 2 years to get that number and work every day of those 2 years getting signatures. At a theoretical constant rate, you need to gather about 550 signatures each day. That's a lot, so let's say you gather a posse of 10 volunteers to help out. That's still 55 signatures each day. For 2 years, every single day.

In other words, it helps out a lot to have an organization, strategy and funding. But then once you get into the organization game you have to navigate internal politics. I'm not kidding that people will spend literal years of their life doing this as a full time job for causes they believe in.

You also have consider the various "gotchas" the legislature his built into the process. For instance the petition you have to print must abide by various requirements. Unfortunately the newspaper printing business has taken a few hits in the past couple of years, and so it's actually pretty expensive printing thousands of specialized, one-off petitions for people to sign.

I'm not saying all this to discourage you. Getting initiatives on the ballot is a super admirable and great way to participate in democracy. The whole reason for them is for us to get around the BS in the legislature to get stuff we want implemented. But it's worth knowing what it takes and what it means for you personally.