Complex sentence: Sullivan studied law in his brother's law practice in Durham, New Hampshire, and was eventually admitted to the bar in Massachusetts, where he established a practice first in Georgetown, then shortly afterward in Biddeford, where he was the town's first resident lawyer.
Split into simpler sentences: Sullivan studied law in his brother's law practice in Durham, New Hampshire, and was eventually admitted to the bar in Massachusetts. He established a practice first in Georgetown, then shortly afterward in Biddeford, where he was the town's first resident lawyer.