The relationship between Santana and Brittany starts when they both attended Hogwarts School of Witchcraft and Wizardry. Santana, who's sorted into Slytherin and Brittany who's sorted in Hufflepuff differences one year. As soon as they met, they become close and started off as best friends.
Santana, who've liked Brittany for the very first moment, couldn't figure her feelings out since she was also kown as the coldest bitch in school. Santana was deeply in love with Brittany, and her feelings for her was getting difficult for others when they were seem much closer as normally friends. They kissed, hugged and laughed pretty much as they were also holding hands each time they were together. The students around them guessed there was something more going on, but didn't showed any interest about it.
Brittany and Santana had a 'best-friends with benefits' arrangement. This was hinted at in various instances in 1992. Brittany, who was always open about her bisexuality, didn't care much what other people would think about her, since she knew most of the people would think she sucked or was very childish. Their relationship didn't become official since the start, as it was more like a friends thing to do for them.
When Santana finally admit her love for Brittany, they are making out in the dungouns and became secret official in 1993. They got snapped by Severus Snape, who calls the two back to sleep when it's night time. They become official in public with the support of their friends. They make love for the first time in 1994 when they sing Like A Virgin.