You can start with the online Catholic Encyclopedia entry. Also, if your parish, school or college library has it, Butler’s lives of the Saints.
Do a Google search for Saint Teresa of Jesus (her professed Religious name - Avila was where she was born.) or Avila. Should give you loads of stuff.
Saint Teresa was from a rich merchant family who were Jewish Converts. Her grandfather converted to Catholicism in the 1470’s.
her three best known books are:
The Way of Perfection 1569
The Interior Castle 1577
The Book of Her Life 1566
I wouldn’t start with the books unless you have a few months as they require a quite slow, contemplative approach to the reading. They are very deep veins of spiritual richness and will greatly bless and enhance your Catholic faith.
There are a number of good biographies and studies of Saint Teresa, her life, her mission, her spirituality. She was a mystic and experienced many visions of Our Lady and Our Lord but she was also very grounded in the everyday practicalities of managing a religious community. In her book The Way of Perfection, a treatise on prayer there are numerous shrewd comments and observations concerning the practicalities of religious life and living in community as well as notes on prayer. For St. Teresa it was all one.
She reformed (some say ‘refounded’) the Carmelite Order. Her 'branch of the Carmelite family are the Discalced ( a word meaning ‘shoeless’, another name for poverty) Carmelites, also known as the Teresian Camelites. She founded over 18 communities during her life as well as being a spiritual director in her own right and being a very good Prioress and novice mistress. For a period St. Teresa’s confessor and spiritual adviser was St. John of the Cross, whom she advised to found the Discalced Friars.
Naturally, as an ‘Aspirant’ of the Secular Discalced Carmelites she is my very favourite Saint. Her Feast day is October 15th.