In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
If you intend to marry this girl then propose to her and tell her that you would like her to become a Muslim.
If you do not intend to marry her then tell her that Islam prohibits free mixing of opposite gender and that you had erred in communicating with her. Tell her you are sorry and ask her to forgive you for your mistake.
Inform her that you have repented and refer her to a female Muslim or to reliable female web site such as www.idealwoman.org for Islamic Guidance.
And Allah Ta’āla Knows Best
Mufti Saeed Ahmed Golaub
Islamic Solutions
Jamaica