All goood advice.   Above all, be yourself.
To extend vocal range, practise singing familiar songs in different keys. To get words in to your head and to help diction, practise songs twice as fast as usual.
Even if you usually sing with accompaniment, do some   unaccompanied practice. This can help natural timing, rhythm and phrasing.
When rehearsing a song, make notes of when to breathe, with attention to meaning and phrasing   - in other words, don't mess up a good sentence or phrase with a badly placed breath. (Read about using your stomach and breathing in quietly   - every day I hear records on the radio with dreadful audible breathing.......).
Leisurely (i.e slow breast-stroke or back-stroke) swimming is good for breathing. Go through a song in your head whilst swimming along and breathe at what you think are the approprate places. It can be done...
Nothing to do with actual vocal technique, but try reciting songs to help with aforementioned meaning and phrasing etc.

And.... be yourself