Tips for Building Tanks

I have been playing mainly as a support for so long, after playing All-For-One since yesterday, I realize that I need to start expanding on other champions because I have never played so many of them before-

I have played ADC and Mid champs before so I have a rough idea how shit works but tank? Yeah that's completely out of my scope. So as a complete noob in playing tank characters like Mundo and Leona, I humbly ask for y'all experts' advice on what should I build for tank champions in general?