First, you need a forum. Usually, this means paying for web hosting and setting one up, but some sites offer free forums already set up for you (but from what I've seen, these are limited in what you can do). Or you could host yourself, which involves setting up a web server.
Then, after setting up your forum, install the tapatalk plugin/mod for the forum software you used then sign up on tapatalk.com. this is the easiest part.
For an app, there are several sites you can use to make "apps" that are simply just a webview of whatever page you hard-code in. (Like the vmroms app and the app for my forum/minecraft server)