For modeling you submit your pictures to agencies and they will call you if they think you have potential.
Here is a list of agencies in NY that represent men
https://models.com/agencies/Modeling/United-States/New-York/Men
Apply to as many as possible - you might have to apply to 10 or more to get even one callback. And if you apply to 10 or more and don't get any callbacks then you have to accept you don't have the right face or body to be a model
For acting you start by taking lessons - there are so many acting schools and classes in NYC
https://www.timeout.com/newyork/theater/best-acting-classes-in-new-york
http://www.backstage.com/advice-for-actors/resources/six-classic-nyc-acting-schools/
https://www.timeout.com/newyork/theater/best-acting-schools-in-nyc
Then you start to get experience by auditioning for amateur productions like community theatre, small theatre companies, etc