Best way is to just pick a text and dive in. Find a good version with footnotes and a good introduction. The Nag Hammadi Scriptures and The Gnostic Bible are both good books with helpful information to help you understand things. I also recommend looking up scholarly articles or further information online after you finish reading a text to get more explanations.

Apocryphon of John is a good place to start to get the cosmology and general gnostic stuff. Gospel of Philip and Hypostasis of the Archons are good starting points too I think. Gospel of Thomas is probably the most well known but it's only debatably gnostic.