For many singers, and maybe for you, the biggest challenge is finding the starting note of a song. If the song is started correctly, everything falls into place. But if not, all kinds of trouble ensues. You can gain confidence that you will start on the right note if you follow these steps and develop the skills that go with them.
1) Determine what the tonic note sounds like. The tonic note is the home note, the anchor note, the note that identifies what key you are singing in. If you are singing a cappella, this note will probably be played on a pitch pipe or other instrument. If you are singing by yourself, you can often choose the tonic note by picking any note that is appropriately high or low in your range.
If you are singing with accompaniment however, you need to identify the sound of the tonic note from the accompaniment. You can often do this by listening to the end of a phrase. If it sounds like a spot in the melody could be the end of the song, you can be sure that the note is the tonic. Virtually all songs end on the tonic note, and if you imagine a note that would conclude the song, it's likely to be the tonic.
2) Locate the tonic on the sheet music. There are simple and well-defined rules for this. For example, if you look at the key signature at the start of the song, if there are two or more flats, they are in a particular order from left to right. If you start at the right and count to the second flat (from the right), that flat will be on the same line or space as the tonic note.
3) Find your first note on the sheet music and how it relates to the tonic note. Sometimes your first note will be the tonic. That makes it easy. More often it is another note of the scale. You need to realize that the lines and spaces on the staff represent consecutive notes of the musical scale. If the tonic is note number one of the scale (and it is), you can count up or down from the tonic to find what number your starting note is. (Remember that note 8 is the same as note 1.) In this way you can identify the number that corresponds to your starting note.
4) Identify in your mind the sound of your starting note. This gets easier with practice and training. If it's note 2, it's just one scale step above the tonic. If it's note 5, you can imagine it by singing the first notes of "Born (1) Free (5)", or imagine the first notes of "Here (5) Comes (1) the Bride". There are other melodies that will help you to learn the sounds of the other notes of the scale. Alternatively, you can just sing up or down a scale until you come to your note. In this way, you can mentally hum or sing your first note.
5) Sing the note you're thinking. This may seem like a complicated and involved process. But as you do it many times, it becomes second nature. By using this list of five steps, you can identify the steps that you can do easily and the ones that need more practice. Focus your efforts on the ones that need more work. Soon you'll be starting every song with confidence every time.