I don't know how close you are to your Grandmother, but take time to really get to know her and what her spiritual beliefs are. Spend a lot of time listening. Do not try to "convince" her of anything (that is the Holy Spirit's job) but when the time is right, tell her why you are a christian and what Jesus has done in your life. The stronger the relationship you have with her, the easier it is to talk about the things closest to your heart. And as Oreo suggested, always prepare yourself and her heart with a prayer.