These four powerful mantras are the first and initial mantras of the four Vedas.

1. First mantra of Rigveda >

अग्निमीळे पुरोहितं यज्ञस्य देवमृत्विजम् । होतारं रत्नधातमम् ।।

agnimile purohitam yajnasya devam rtvijam
hotaram ratnadhatamam.

I invoke and worship Agni light of life, self- refulgent lord of the universe, foremost leader and inspirer, blazing light of yajnic creation, high-priest of cosmic dynamics, controller of natural evolution, and most generous giver of the treasures of life.

2. First mantra of the Yajurveda >

इषे त्वोर्जे त्वा वायव स्थ देवो वः सविता प्रार्पयतु श्रेष्ठतमाय कर्मण आप्यायध्वमघ्न्या इन्द्राय भागम्प्रजावतीरनमीवाऽअयक्ष्मा मा व स्तेनऽईशत माघशँसो धु्रवाऽअस्मिन्गोपतौ स्यात बह्वीः यजमानस्य पशून्पाहि ॥

Ishe tvorje tva vayava stha devo vah savita prapayatu sresthatamaya karmana apyayadhva maghnya indraya bhagam prajavati ranamiva ayakshma va stena isata maghasamso dhruva asmin gopatau syata bhavir. yajmaanasya pasun pahi.

Be vibrant as the winds and thank the Lord Creator, Savita, for the gifts of food and energy, light and life, for the body, mind and soul. Pray that you dedicate yourself to the noblest action, yajna, and play your part in the service of the Lord. Be blest with the best of health and wealth in plenty, cows, healthy, strong and fertile, sacred, not to be killed. No thief to rule over you, no sinner to boss over you ! Growing in power and prosperity, be firm and loyal to this Lord of the Nation and protect the wealth and honour of the yajamana.

3. First mantra of Samaveda >

ओम् अग्न आ याहि वीतये गृणानो हव्यदातये। नि होता सत्सि बर्हिषि ॥

agna a yahi vitaye grnano havyadataye. 
ni hota satsi barhisi.

O God, we realise Thee, as Thou art Luminous, Pervading and Giver of enjoyable objects. Thou art Worthy of adoration, present in the world and our soul, like a Hota in the Yajna.

4. First mantra of Atharvaveda >

ये त्रिषप्ताः परियन्ति विश्वा रूपाणि बिभ्रतः ।
वाचस्पतिर्बला तेषां तन्वो अद्य दधातु मे ॥

Ye trishapta pariyanti vishva rupani bibhratah
vacaspatirbala tesaam tanvo adya dadhatu me.

May God ever assign to me the strength and powers of those twenty one objects, which sustaining the animate and inanimate creation, are wandering round.