There are three cruise terminals in the port of New York: Manhattan, Brooklyn and Cape Liberty (which is actually in Bayonne NJ rather than NY City).
Which one are you asking about? While there will be taxis available at all three the amount of time to get to LGA will vary, with Cape Liberty taking longer than the other two. If you're coming into Cape Liberty you don't want to use a taxi at all to go to LGA...Uber or a car service would be much better.
In Manhattan you should cross 12th Av and pick up a taxi there, as @navybankerteacher said, but in Brooklyn the taxis will be available within the confines of the cruise port grounds. At either cruise terminal even doing self disembarkation it may take a few minutes to actually get a taxi as many others will be doing the same. 30 minutes drive time to LGA from Brooklyn or Manhattan is an absolute minimum, but never count on that. Allow at least an hour.