As well as building your range it's also important to focus on your articulation.
It is important to be able to sing words clearly and that the effect of pronouncing a word does not affect the quality of your voice.
Slowly sing the tune of 'Do - a deer' from the Sound of Music to 'aah', making sure you sing really evenly and that the sound you are producing is consistent.
Next, add the words: try and keep the consonants really smooth and the vowels clearly formed. 'Do - a deer, a female deer; Re - a drop of golden sun...'.
See how much of the song you can sing in one breath.
When you do breathe make sure it is in a logical place in the text - would you breathe at that point if you were speaking it?
Keep listening to yourself at all times.