I just launched ceev.io a few weeks ago and was wondering the same thing. I had everything available for free at first, but wanted to monetize it a bit.

I worked on a few new features - 2 new resume templates and two new editing options - and added those under a paywall. It wasn't much, but I also promised people who upgraded new exclusive features weekly. At least for Ceev.io, that worked pretty well. I got many more sales than I was expecting, and it seems people who bought it right away were not disappointed even though there weren't many paid features.

So maybe start small - add a few features for paid users, then just keep adding features and raising the price. Let users know that by paying, they will continue to get exclusive features, and they are also supporting the project/website/community. For many people, that will be enough to start, and as you add more paid features, more people should think it's worth the price and will be more willing to pay.