This is where the research of a man named Erwin Chargaff came into play. By using Chargaff's rule two scientists named Watson and Crick discovered that Thymine paired with Adenine and Guanine with Cytosine. They also discovered that a hydrogen bond was obtained when the bases were paired together in this way. Pictures on how Thymine and Adenine bond together. In Geometry complimentary angles can be placed side by side to form a right angle.
Base pairs are like complimentary angles.
In a DNA strand Adenine is always paired with Thymine, and Guanine is always paired with Cytosine. Links I suggest that you read over the History of DNA Research or at least part of it then read about Replication.