How to interface with an API from android?

I built an android app and have built a corresponding node.js back-end to store data from it. I've got some API end-points created along with methods to save and retrieve data from a MongoDB.

I understand how to make https requests from java/android, but I'm not sure how things should ideally be set-up and structed. Is it best practice to create a new package for api related things? Like for example, a package like:

API Package
    Class containing GET requests
    Class containing POST requests

These classes might contain methods like:

public static String getItem(int item_id) {
    //HTTPS boiler plate
    //HTTPS GET request and process data
    return result;

Or is it best to create a class to handle and make HTTPS requests and then implement all of the required API methods in a single, separate class?

There are several popular libraries out there that you can check it out like Retrofit or Android Volley. I recommend you not to write the code to make requests to the backend yourself because things like parsing JSON data shouldn't be done manually as everything will be hard to manage when your project grows.
To structure the code, it depends on personal preference. I personally use Retrofit so I keep all network requests in an interface. For example:

interface APIs {
    @GET("api/user")
    Call<Users> getUsers();

    @GET("api/comment")
    Call<Comments> getComments();

    @GET("api/photos")
    Call<Photos> getPhotos();
}

But if you want to learn how things should be set-up and structured nicely, I suggest you read about the Android architecture component and Clean Code architecture where code logics are kept separately and grouped in a manageable way.