How do I sing high notes without straining?
Work just below your top, not at it. Pushing more air at a high note usually makes it crack; lighter, slower slides into the top notes build them more safely.
A useful rule: only move your top up after several sessions where it held steady. If the top note wobbles or tightens, step back a semitone. Singsteady applies exactly that rule, and stops if anything hurts.
More questions: Is there an app that checks if I'm singing in tune? · How accurate is an online pitch or vocal range test? · What is a cent in singing? · Can you learn to sing in tune? · How do I find my vocal range?