I grabbed my trusty "Companion to Narnia" and was shocked to find no pronunciation guides.
Which names are giving you difficulty?
I say Jadis like "JAY-dis" (ay as in hay).
Charn with a ch like church.
Aslan like "AZ-lan" Many people, esp. in Britain say "ASS-lan".
I loved the Lord of the Rings for twenty years before re-reading the appendices and realizing I had mispronounced just about every name in the book!
I'm probably mispronouncing these, too.