A catechism is a summary and exposition of the teachings of the Faith.   Below are some popular online catechisms.  Each  has the same information but presents it in slightly different ways…find the one that you like best.

Antiochian Summary of Beliefs
Concise explanation of major beliefs and how the Faith impacts select issues of modern society

These Truths We Hold From St. Tikhon’s Monastery
Well organized and thorough presentation of basic Orthodox teachings

The Longer Catechism of St. Philaret of Moscow
Very extensive and in the question/answer style of the Catholic Baltimore Catechism