Answer

If you mean the 108 names/list, the first Bhagwan is usually Shri Ajhara Parshwanath in the commonly printed alphabetical order. ( jainqq.org)

If you mean the main deity of the pujan, then it is Bhagwan Parshvanath himself, and the ritual is specifically for his worship. ( jaingpt.org)

Explanation

In different Jain books or local vidhi, the order of the 108 names can vary a little. But in the alphabetical listing shown in Jain literature, the first entry appears as A1 Ajhara. ( jainqq.org)

Spiritual Understanding

The purpose of 108 Parshwanath Pujan is devotion to Bhagwan Parshvanath through his many sacred forms and names. The exact order is secondary; the bhavna of श्रद्धा and bhakti is the main thing. ( jaingpt.org)

Takeaway

So, first in the 108-name list: Ajhara Parshwanath; main worshipped Bhagwan: Parshvanath. ( jainqq.org)