To improve the odds of fostering that next big thing in New York, executives at tech start-ups, big tech companies and venture firms are creating a new policy and advocacy organization, Tech:NYC. The nonprofit will be formally announced at an event this week. Tech:NYC has no set agenda yet. But its approach, said Julie Samuels, the executive director, will be to work early with city, state and federal officials on issues that affect tech companies, before laws are passed. Some broader issues like taxes, schools and affordable housing also help determine whether people want to work and live in a city.