There are 4 nitrogenous bases, or molecules, that make up DNA: adenine, thymine, cytosine, and guanine. They will actually pair up to make the strands of DNA. However, they specifically pair up, thus making the complementary bases. For DNA, cytosine only pairs up with guanine (C with G). Adenine only pairs up with thymine (A with T). So, you would only find these pairs in DNA.
However, you wouldn't find only C and A on one side, with G and T on the other. All four bases exist on each side of the double helix for DNA. You might have one side of DNA reading ACTGTGA. So, then, following "C with G" and "A with T," the other side of the DNA would be TGACACT.
I hope this helps.