Security is the one job on this list where the paperwork comes before the search. New York requires a license, and no legitimate employer can hire you without it. Get licensed first and the job search becomes easy, because demand in Brooklyn consistently outruns the supply of licensed guards.
You need a New York State security guard registration from the Department of State. That means the 8-hour pre-assignment training course, then the 16-hour on-the-job training course within your first 90 days of employment. You will be fingerprinted and background checked. Keep the certificates -- employers ask for the physical paperwork.
Armed positions require a separate 47-hour firearms course and a pistol permit, which is a much longer and harder process in New York City. Start unarmed. Most Brooklyn openings are unarmed anyway, and a year of clean unarmed experience is what gets you considered for the armed roles later.
Apply to the staffing agencies, not just individual buildings. Large security firms hold the contracts for most Brooklyn residential buildings, hospitals, construction sites and retail, and they hire continuously. One application to an agency can put you in front of dozens of sites.
Decide what kind of post you want, because they are very different jobs. Residential door work is quiet, indoors and social. Construction site work is overnight, cold in winter and alone. Retail loss prevention involves confrontation. Hospital posts involve de-escalation. Agencies will offer you whatever is open, so know what to turn down.
Your record matters here more than elsewhere. The state background check will surface a criminal history, and some convictions bar licensing. If you are unsure where you stand, find that out before paying for the training course.