Suppose the first nucleotide on a codon is adenine. What will the first nucleotide be on the corresponding anticodon?

The codon is found on the messenger RNA molecule and is complementary to the anticodon on the transfer RNA molecule. The base pairing in RNA molecules is:

Adenine with Uracil (because RNA molecules do not have Thymine)

Cytosine with Guanine

This means that the first nucleotide on the anticodon will be uracil.