Answer

After the Navkar Mantra, there is no compulsory next mantra in Jain Dharma. But for regular practice, many people learn these next:

• Logassa Sutra — very common for daily prayer
• Uvasaggaharam Stotra — very beloved devotional stotra
• Icchami Khamasamano — useful for humility and seeking forgiveness

If you want one simple next step, I would suggest: learn Logassa Sutra after Navkar Mantra.

Explanation

The Navkar Mantra is the most important and complete mantra in Jainism. After learning it, the next recitation depends on your purpose:

• For daily worship: Logassa Sutra
• For devotion and peace: Uvasaggaharam Stotra
• For repentance and humility: Icchami Khamasamano

So there is not one fixed rule. The best next learning depends on your sadhana.

Spiritual Understanding

In Jain Dharma, the purpose of mantra and stotra is not only recitation, but also:

• purification of mind
• remembrance of the Arihants and Siddhas
• development of humility
• reduction of attachment and ego

That is why Navkar Mantra is first, and then other sutras and stotras are learned gradually.

Takeaway

Best simple answer: Learn Logassa Sutra next. If you want, after that you can also learn Uvasaggaharam Stotra and Icchami Khamasamano.