Lord Agni is worshiped in the first Sukta of the first Mandala of Rigveda. His sages are Madhuchhanda and Vaishvamitra. His used verses are Gayatri. There are total 9 mantras in the first sukta.
ॐ अग्निमीले पुरोहितं यज्ञस्य देवमृत्विजम्। होतारं रत्नधातमम् ॥ 1 ॥
The first mantra of Rigveda is dedicated to Agni Dev. Rishis say in their prayer – the priest of Yajna, the deity, the Rishi and the deities are invoked and we worship the fire which gives the benefit of Yajna to the priests.
अग्निः पूर्वेभिर्ऋषिभिरीड्यो नूतनैरुत। स देवां एह वक्षति ॥ 2 ॥
Rishis say – Agni praised by the ancient sages is praised by the sages even in this period. That fire should invoke the deities in this yagya.
अग्निना रयिमश्नवत् पोषमेव दिवेदिवे । यशसं वीरवत्तमम् ॥। 3 ॥
Sages say – Agni, the giver of wealth and fame, is the one who gives sons and grandsons and valor (heroic men) to the hosts (humans) every day.
अग्ने यं यज्ञमध्वरं विश्वतः परिभूरसि स इद् देवेषु गच्छति ॥ 4 ॥
Rishis say – O fire capable of protecting everyone! The yagya by which you yearn for the gods living in heaven, you cover that yagya from all sides without any hindrance.
अग्निहोता कविक्रतुः सत्यश्चित्रश्रवस्तमः । देवो देवेभिरा गमत् ॥ 5 ॥
The sages say through this mantra – Oh fire, who comes with the deities in this yagya! You are the giver of sacrifice, the inspirer of the combined power of knowledge and action, the form of truth and the unique form.
यदंग दाशुषे त्वमग्ने भद्रं करिष्यसि। तवेत् तत् सत्यमंगिरः ॥ 6 ॥
Rishi says – O fire for the welfare of the sacrificer! The wealth, housing, children and prosperity etc. that you provide, you get that through the deeds done in the future.
उप त्वाग्ने दिवेदिवे दोषावस्तर्धिया वयम्। नमो भरन्त एमसि ॥ 7 ॥
The sage says through this mantra – O fire! Your worshipers, we worship you with the best intelligence to get your company, we praise you continuously day and night.
राजन्तमध्वराणां गोपामृतस्य दीदिविम् । वर्धमानं स्वे दमे ॥ 8 ॥
Rishi says – O Radiant Fire! You are the savior of Yagyas, the illuminator of truth and the one who is going to get growth in the place of Yagya.
स नः पितेव सूनवे ऽग्ने सूपायनो भव । सचस्वा नः स्वस्तये ॥ 9 ॥
The sage prays to Agni in the last mantra of the first Sukta and says – O Agni! As a son easily receives his father, in the same way you may be received by us happily without any hindrance and may you always be near us for our welfare.