mocqups, mockflow, flinto, balsamiq, marvelapp, invision and many others, with a single google search you can find plenty of such app wireframe creation and prototyping tools.
Some are good for even laymen, some are better for people already know some designing.

If nothing works, then drawing on plain paper and sharing with designer/ developer would be sufficient as well, though in this case you'd need to rely on designer/ developer's creativity on the top of your rough designs. We've many clients who just gave us an idea and couple of sketches depicting app's flow and we made whole good apps out of it.

We've compiled a list of 10 mobile mockup creation tools at this link https://www.agicent.com/blog/a-look-at-10-of-the-best-mockup-and-wireframing-tools-for-developers, I hope this helps you more.