Hello and welcome! You'll certainly learn plenty about GIS just browsing around here, but do be sure to check out Esri's other training materials, too. There's a MOOC going on right now, so if you wanted to learn about the App side of AGOL, it's a great place to start. https://www.esri.com/training/catalog/57660ed7bb54adb30c9454a3/do-it-yourself-geo-apps/
