Opening a database is database specific, here there are examples for some databases.
Sqlite 3
file := "path/to/file"
db_, err := sql.Open("sqlite3", file)
if err != nil {
panic(err)
}
MySql
dsn := "mysql_username:CHANGEME@tcp(localhost:3306)/dbname"
db, err := sql.Open("mysql", dsn)
if err != nil {
panic(err)
}