Firstly, I'm a 15 year old male (soon to be 16) and since my voice changed because of puberty I couldn't sing as well as I used to. (I was in a church choir and one of the nuns told my mum that when she listened to me sing she forgot about her troubles and she loved my voice - which was such a nice compliment to hear).
I've completed a singing course online and I personally think I'm not too bad of a singer but I have troubles with some of the higher notes for example Ariana Grande's Break Free, the part where it goes ''This is, the part where I say I don't want ya, I'm stronger than I've been before". The words ''I'm stronger than I've been before'' are quite high for me and sometimes my voice strains / my throat tightens and my larynx rises a lot meaning it sounds stressed.
(I can sing it with less stress but then it sounds breathy / loud whisper type sound) So the first part of the question is how can I gradually eliminate tension when I sing higher notes without the cost of volume? Is there any exercise that helps with this?
Secondly, I've recently fallen in love with 'Emotions' by Mariah Carey and I've watched Adam Lopez sing it with beautiful whistle tones which sound a bit airy but still sound similar to Mariah's. I've tried to mimic this but all I've gotten was the sound of something along the lines of a whelp or a dragon screeching. Once again, how can I achieve whistle notes and are there any exercises that you recommend?