A date would help. On the average, women have their first child at age 20 and their last child at 40. If you are the eldest son of the eldest child and are now 18, your grandmother was born 58 years ago. If you are now 60 and the youngest son of the youngest child, your grandmother was born 140 years ago. That is an 82-year range. Your grandmother's spouse's name would help too.
If your grandmother had a Social security number, the SSA will sell you a copy of her application for $27. It will list her parents' names, which gets you back another generation.
Most of the Casselberry/Castleberry/Castleburg lines go back to Hendrick Kesselberg, who came to Pennsylvania in 1683 and started using different spellings. He died Henry Castleberry. Once you get back to 1850 or so you'll probably find lots of research and a clear shot to Henry.