1) PICK A NOTE.
There is no need to slide up and down the scale like a vaudeville trombone whenever a note is held for more than one beat. It's cheap, it's annoying, and it's cheesy. You never heard Aretha Franklin pull that crap, did you? Did you ever hear Sam Cooke do that kind of BS? Or Otis Redding, or Diana Ross, or Wilson Pickett? No. But then, they were the kind of singers that you would call, um, what's the word, good! They were good. They didn't have to resort to that cheap wankery to try to show off their vocal control, they just sang really, really well. You know who does do that? Hacks. American Idol contestants. Mariah Carey. It's weak and stupid, just knock it off.
2) Not every song needs to be remade. If it was good the first time, you're only going to suffer in comparison. If it sucked the first time, your version probably isn't going to suck any less. Now if you are a young Whitney Houston and you want to tear up Dolly Parton's "I will always love you," by all means go ahead! If you are a young Whitney Houston, you are going to sing the hell out of that song, and raise it to a whole new level. But you are probably not a young Whitney Houston, so try writing a new song.
3) If you do write a new song, try giving it a tune. Or a Melody. Maybe then you won't have to glissade over every note like Sideshow Mel's slide whistle.
4) Turn off the auto-tuner. I can not emphasize this enough. If you can't carry a tune on your own, maybe singing isn't for you.
And there is nothing clever about turning the auto-tuner up so that you sound like a robot. You're probably too young to remember Midnight Star, but this is from 1983:
And they probably weren't the first to do the robot voice. Either way, it's not original, it's not clever, and it's irritating as hell. Stop it.
Addendum: Of course they weren't the first, I totally forgot about Kraftwerk