Two skills that are needed by aspiring musicians are Sight Singing and Ear Training. But they are really two perspectives on the same skill. You can't master one without the other, and competence that you build in one is directly transferable to the other.
I should clarify, before an academic musician protests, that I'm talking about Melodic ear training. One facet of ear training, the Harmonic variety, deals with identifying chords or intervals when multiple notes are sounding at once. Melodic ear training, which relates to sight singing, is identifying the notes of a single melody that is being played or sung.
Using the helpful metaphor of language learning, sight singing is like being able to speak a language, while ear training corresponds to being able to hear and understand a language. The skills are obviously related, but somewhat different; one is passive, one is more active. But you need to work on both of them in order to become fluent.
Sight singing requires identifying which line or space on the music staff indicates "Do", the tonic note, and then being able to produce sound corresponding to other notes based on their relation to Do. To do this, one must be able to hear the note relationships in one's mind, and this is the core skill of ear training. Ear training is essentially identifying the tonic pitch from the melodic context, and then being able to identify the other notes based on their relationship. If you can hear a note in your mind, you can reproduce it.
Of course, the first challenge in either Ear Training or Sight Singing is to identify the sound of the tonic in a musical context. This is made easier for the beginner when he realized that almost all melodies end on the tonic note, Do. So if you're trying to identify the starting note of a melody, you can imagine the song ending and then starting again. Does it start on the same note it ended on? If so, it probably starts on Do. An example is "My Country, 'Tis of Thee" ("God Save the Queen").
Once you're able to identify the sound of Do, it's fairly straight-forward to learn the sounds of other notes in the same tonal context by identifying familiar songs that either start on the note you're trying to learn, or start on Do and move immediately to the note in question.
An example is the note "Sol" (note number 5 in the scale) below Do. You can use a song like "Born Free" to go from Do to that note, or a song like "Amazing Grace" that starts on that note. There are similar examples for most notes of the scale. For learning to recognize Sol above Do, you might consider the tunes to "Twinkle, Twinkle, Little Star" (Do - Sol), or "London Bridge is Falling Down" (starting on Sol).
Developing a fluency for identifying notes that you hear (ear training) will help you a lot when it comes time to produce the appropriate notes yourself in the process of Sight Singing. Practicing one of these skills will automatically improve your ability in the other.