Build these solid foundations and no song will be too hard.

Support – (understanding and utilising correct abdominal [or diaphragmatic] support)
This is the key to great strength, stress management (relaxation), stamina, range, and power.

Breath management – (understanding how to correctly use your air)
The over-use of air is one of your biggest enemies as a singer. It dries out your cords and causes hoarseness, strain and fatigue it also often causes you to sing off pitch.

Good open throat technique – (correct tongue and jaw placement)
Understanding good vocal tract shaping (the shaping of vowel sounds)…  in order to not “over enunciate” or “over-shape” the vowels to control their size and shifting within the vocal tract. This helps leaps and bounds with your stamina, range, and tone.

Posture – (learning to build a “strong tower” of consistency with good posture and rib cage expansion)
This frees up the chest neck and throat from strain and stress, and helps with your breath management and diaphragmatic support.

The psychology of singing – (not allowing the mind to “freak you out” so you can sing with ease and freedom)
This largely comes down to confidence, which largely comes out of mastering the first 4 points. And when you have the “psychology” of singing down pat, it helps you tremendously… stopping you from locking down your vocal cords (which you’re prone to doing when you’re not confident) which causes pinching, squeezing, and literally choking off of your cords when you’re ascending a line or passage in a song.