Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 1: Contents of the Rāmāyaṇa Summarized
Text 1.1.57

सुग्रीवाय च तत्सर्वं शंसद्रामो महाबलः।
आदितस्तद्यथावृत्तं सीतायाश्च विशेषतः॥

sugrīvāya ca tat sarvaṁ śaṁsad rāmo mahā-balaḥ
āditas tad yathā-vṛttaṁ sītāyāś ca viśeṣataḥ

sugrīvāya ca = Sugrīva; tat sarvam = about everything; śaṁsat = told;  rāmaḥ = Rāma; mahā-balaḥ = of great strength; āditaḥ = from the very beginning; tat yathā-vṛttam = that had taken place; sītāyāḥ = about Sītā; ca = and; viśeṣataḥ = especially.

Rāma of great strength told Sugrīva about everything that had taken place from the very beginning and especially about Sītā.

Nārada Muni points out that Rāma revealed confidential details [about Himself and His situation to Sugrīva] to become friends with him. Mahā-balaḥ indicates that Rāma was forbearing—He concealed His dejection caused by remembrance of those events. He told Sugrīva about everything, that is, from His birth. He specifically told Sugrīva about Sītā and how she had been kidnapped by Rāvaṇa and so on. Because He was searching for her, He specifically spoke about her.