This mantra is from the Yajur Veda (36.iii) and is one of the oldest known mantras. It asks that our intellect be guided towards knowledge.
Om bhur bhuvah svaha
Tat savitur varenyam
Bhargo devasya dhimahi
Dhiyo yo nah prachodyat
Earth, Sky and Heavens
That divine Sun worthy of my worship
I meditate on the divine effulgence
and pray that my intellect be guided towards knowledge.
OM BHOOR BHU-VAH SU-VA’-HA
TAT SA’-VI-TUR VA-RE-E’-NYAM
BHAR-GO’ DE-VA-SYA’ DHEE-MA-HI
DHEE-YO YO-NAF’ PRA-CHO-DA-YA-A’-TUH
Un-notated syllable = neutral note or SA
Underscored syllable = ½ note above or flat RE
Hyphened note = note below or NI