Answers so far sound pretty accurate. As a securities attorney that's advised hundreds of startups, here are my tips:
First, connect with other successful, more advanced founders. They can advise you on the mistakes they made at your stage and make referrals.
Second, don't focus on engaging experts before you've validated that investors care about a better plan, financial model, or valuation. If you've got something valuable, prospective investors will tell you as you start the early conversation.
Finally, engage an attorney to ensure that you're not violating securities laws. A few hundred dollars for an in-depth conversation might save you hundreds of thousands later.