If the sequence of the 5'-3' strand is aatgctac, then the complementary sequence has the following sequence:
3'-aatgctac-5'
3'-catcgtaa-5'
3'-ttacgatg-5'
3'-gtagcatt-5'

3'-TTACGATG-5'

Explanation:

The DNA sequence is in couples where A is united T and G is united C, so what we do is replace in the complementary sequence the original nucleic acids, remember A-T, G-C.

5'-AATGCTAC-3'

3'-TTACGATG-5'

To remember: the DNA sequence is 5'-3' in one sequence and its complementary is inverse, what means it goes 3'-5'.

Also the T-A is united by 2 hydrogen bonds, and G-C is united by 3 hydrogen bonds.