Metaculus is a reputation-based site for soliciting and aggregating predictions. It was founded in November 2015 by astrophysicist Anthony Aguirre, cosmologist Greg Laughlin and data scientist Max Wainwright.
The official Metaculus tutorials are suitable for beginners as a way to introduce forecasting, probability distributions, and a basic understanding of how to behave under uncertainty.
Tutorial #1 is over-explaining for those with a basic understanding of the forecasting, but still a good introduction to using Metaculus specifically.
For those familiar with forecasting, tutorials #2 and #3 are more useful as a (small) amount of calibration practice and building familiarity with the Metaculus interface.
A video introduction to Metaculus is also available.