Who taught us the rituals of Muharram? Did any Imam teach us or they are just the fabrication of forefathers whom we now blindly follow? If the latter is true, what is the difference between us and the non-believers who insisted on following their ancestors and no one else?

The basic rituals of mourning were taught by the Imams.
The first error-free Imam who taught this was Imam Zayn al-Abidin (A.S). He was followed by Imams Sadiq (A.S) and Reza (A.S). Both held association – majalis – during the first 10 days of Muharram to mourn and weep over the great sacrifice made by Imam Husayn (A.S).