Design mock-up question

I'm new to this sub so my apologies if this isn't the proper venue for this question (promise I read the rules first).

I'm working on a business idea and I need a decent, ideally free, mock-up tool that will allow me to put some ideas into form and share them with others.

My goal is to get everything out of my head and onto "paper", share with my team to get their thoughts, and ultimately hand the mock-ups over to a designer to get professional treatment.

I'm not a designer, so I need something simple but not MS Paint simple.

Really appreciate any help you can offer.

Great design resource

100 Things Every Designer Needs to Know About People (Voices That Matter) Submitted May 08, 2016 at 08:12PM by NastyCamper http://ift.tt/24E980W