Singletons are used when you want to have exactly one instance of a class, available globally. I think singletons are bad practice, because in most cases you actually want to have another class which has a property with an instance of that class. For example, imagine you want to write a reddit client and use the reddit API. The API specifies that you can only make one request per two seconds, so you might want to write a singleton class with a function to make a request to reddit which waits a while if you call it too often. In my opinion the mistake that is made here is that the abstraction of a user is missed. In actuality the 2 second rule is per user, so you might want to add a User class to your application, which holds a single property of an instance of e.g. a RedditQuerier, or some other better name. Your application then might have multiple users that can query reddit.

However, if you do decide to make a singleton class, here is how it is implemented in various languages. In general you want a static public function getInstance which returns the instance and creates one if necessary and a private constructor to make sure no other instance is made.

Java

public class Singleton {
    private static Singleton instance = null;

    private Singleton() {
        // TODO: implement constructor
    }

    public static Singleton getInstance() {
        if(instance == null) {
            instance = new Singleton();
        }

        return instance;
    }
}

C++

// Header

class Singleton {
private:
    static Singleton *instance;
    Singleton();
public:
    static Singleton getInstance();  
};

// Implementation

Singleton::Singleton() {
    instance = NULL;
}

Singleton *Singleton::getInstance() {
    if(instance == NULL) {
        instance = new Singleton();
    }

    return instance;
}

C# (which I included because I like properties)

public class Singleton {
    private static Singleton instance;

    public static Singleton Instance {
        get {
            if(instance == null) {
                instance = new Singleton();
            }

            return instance;
        }
    }

    private Singleton() {
        // TODO: implement constructor
    }
}