Jainism is considered beginningless in its spiritual truth because the soul, karma, and the path of liberation are eternal in Jain दर्शन.

If you mean the historical beginning of the present Jain tradition, it is associated with the appearance of the 24 Tirthankars, especially:

• Lord Rishabhdev (Ādināth), the first Tirthankar of the current time cycle
• Lord Mahavira, the 24th Tirthankar, who revived and strongly organized the Jain path in this era

Simple meaning

• Jainism did not start with one human founder in the ordinary sense.
• It is an eternal dharma revealed again and again by Tirthankars.
• The core teachings are:

- Ahimsa — non-violence - Satya — truth - Asteya — non-stealing - Brahmacharya — chastity - Aparigraha — non-possessiveness

In Jain belief

• Rishabhdev is the earliest Tirthankar of this cycle.
• Mahavira gave the most recent and widely followed teaching in historical times.

If you want, I can also give:

• the origin of Jainism in detail
• the life of Lord Rishabhdev
• or the life of Lord Mahavira