Good resources for designing for non-technical people

I'm part of a team designing an IRC client that needs to be full-featured but also easy to use for people who don't often use IRC (the goal is to create a Slack-like interface, but with all the feature of IRC.)

I'm wondering if there are any resources available for designing-in advanced features into easy-to-use applications. Ideally the client would be suited to both entry level users and experts, with a focus on lowering the barrier for entry into IRC.