Do I need an agreement for my job board?

You have to think of 2 main things:

1. How will you protect your liability.

Depending where you are jurisdiction wise, are you a sole trader or are you trading as a company? You should look to answer this question. If a small project it is not likely worth the cost to set up a company. If more than, say, £4,000 per annum then consider a company (though be aware of monthly costs).

2. How to protect yourself legally

A services agreement with your potential customers should be setup to set out:

The Client giving you a licence to use of their logo/IP/branding for purpose of the service;
Fees and when they're due;
Any limit of liability - e.g. you don't control third parties who may crawl your site and re-display that information. Time limits as well.

The best way to do this would be to have a standard terms of service on your website that basically sets out the above. Maybe consider looking at existing job board websites terms of service to get an idea.

My background: Startup lawyer for SAAS; compliance officer; head of legal and compliance for regulated financial services.

Liability ultimately managed by creating a limited company. TOS useful but not last word. Apparently use of logos OK depending on usage though assume owner retains copyright and can ask you cease. https://www.nolo.com/legal-encyclopedia/when-you-need-permission-use-trademarks.html

Really though. I'm new to this, so any help would be appreciated.