With every organism in the world they all have the same base pairing rules. The rule are that "a" pairs with "t" and "g" pairs with "c". This rule will change when the DNA is copied into MRNA but just just keep it simple.
If I have the DNA code
aatcgtagtcggatat that other side will be the opposite.
ttagcatcagcctata
I hope this makes sense, if it doesn't then let me know. Have a Great day!