Recognize diphthongs both in word, sound, and tongue placement, and practice them in speech and song because you'll need to be
clear and precise in song. I feel like the biggest reason why a lot of modern artists sound mushmouthed and mumble more than sing is because
ebonics they just don't realize this, or don't care to. This is especially important because your soft palette rises when you vocalize, and you'll be sounding out phonics every note. This isn't just for professional singers, this advice is also for your weekly church-goer.
Think another problem modern singers also have is that they
feel they have to have melisma (single syllable running through multiple notes) in their songs or they're not "true" vocalists. Forcing this on yourself when you're having a hard time with pitch or breaths is just going to give you a bad time, and you will
not get better if you can't find your bearings. "Angels We Have Heard on High" is a beautiful song and is fun to do, but inexperienced singers bungle and stumble through it.
If you want to sing and be good at it, you just have to practice as much as you can. Preferably every day for one or two songs, but every other day or so will be fine, too. Best to do it in places with acoustics like your bathroom (shower vocalist master race) so you can hear it bounce off the walls. Go out to some open area with few or no people and sing (I'm a weirdo who used to sing on the swings as a child, and I go on walks and sing because it works for me). Sometimes record yourself so you can get a good feel of how you sound outside of your head,
especially once you discover your head voice.
Also pinch your nose off-and-on while singing. If you don't sound nasally at all, you're on the right track.
You will probably not need more than 10 seconds.
I'm pretty sure I can hold for 15 seconds lol I remember being proud of myself that I could actually sing the end note of "The Bells of Notre Dame" and not pass out. I really should time myself, though, I tend to count "holding" from when I take a breath regardless of how many syllables are in that breath.