i don’t know if they’re free or not—but I doubt it—there are several vocal coaches on youtube that might or might not be helpful. I only sing in flat, so I’ve never tried for a vocal coach

Perhaps you already know these, but if you don’t here they are:

1. Stay hydrated, but avoid alcohol.

2. Try to harmonize with the melody. You can do that by trying to harmonize with the vocals or the instruments. You can tell that you’re harmonizing when your singing forms a simultaneous echo with whatever part of the melody you’re following.

3. Find a melody that you like, and sing along using one vowel for the first run. If that vowel has more than one pronunciation, then use that pronunciation for the first run. and a different pronunciation for the next run, until you reach the next vowel. Repeat.
E.g., for the first run, use a long A (as in cake). For the second, use another pronunciation of A (as in flag), etc.

4. Learn not only the lyrics of a song, but also the applications of the song’s rhythm to the lyrics.

Do vocal exercises before performing.

Vocal coach is a great idea.

There are probably some YouTube videos out there to help you, I would search. Teach you how to fill your lungs, protect your vocal cords, voice exercises, and other tools of trade.

Learning diaphragmatic breathing is also important. Also I suspect there are apps out there to help you get your pitch on key.

Join the Church or school choir.

Keep your chin down. There’s a natural tendency to lift it when reaching high notes that is not helpful.

Practice on karaoke apps! There are a few free ones, with premium paid options. Smule is one that I have used in the past. You can pick songs and record yourself, and even set it to private. You can practice the same song as many times as you want and compare them over time. It helps to use earbuds with a speaker. There is also a smart tv app called karafun that allows you to sync a microphone.