Depending on where you are, this is one thing that's good to post to Craigslist (seriously). Just don't offer "exposure" for their work (inside joke).
If you want to get started on your own, Purdue's OWL (https://owl.english.purdue.edu/owl/) is an excellent resource. Figure out which style guide to use and stick to it; which isn't as important as being consistent.
A higher-level effort is working with a Content Strategist. That will cost more, but a good one will do an audit of your content, work with you on what you want your voice to be, and leave you with workable and simple documentation and guidelines to accommodate future content and expansions.