You need to know the domain you’re consulting about of course, but also you need to know consulting, i.e., how to tell your clients what needs to be done, in a way that is thorough but not insulting, motivating them to actually do what needs to be done, and ideally to continue having you find problems and suggest solutions. You might also need to know how to fix those problems yourself, or at least who you can farm it out to.
If you’re going to be an independent consultant, i.e., your own boss, as opposed to an employee of a consulting firm, you’ll also need to know at least the basics of business, like how to register and maintain a company legally in your jurisdiction (government paperwork and licenses and so on), marketing, sales, filing taxes, customer service, all the stuff that a company does. You don’t need to do everything yourself (in fact it’s advised you don’t, spending the time on client work instead), but you need to know enough about it to assess the competence and pricing of people you farm those bits out to. (If you’re going to stay a single-person company, you can ignore the HR parts, but that’s about it.)
As for resources, that depends where you are; in the USA, try SCORE and the SBA.