You can start with a beginner-friendly Jain book or article on the 24 Tirthankars. On JainKnowledge, these pages may help:

• Stories of all Tirthankars
• Tell me about 24 Tirthankars
• From which book can I start learning about Tirthankars

For a proper book, the most respected traditional starting points are:

• Kalpa Sūtra for a short introduction
• Triṣaṣṭi-Śalākā-Puruṣa-Caritra for detailed life stories of the Tirthankars

If you want, I can also give you a simple reading order for studying all 24 Tirthankars in Jain tradition.