I'm not sure who gave this advice, but they have no idea what they're talking about. Every single comic I've ever started to work on has been the one I was most attached to at the time. You can always redraw, rewrite, or remake it.
When I was 14 I started to draw a comic, I never ended up posting it because I wasn't happy with it and lost motivation. I still love and wanna make that story some other day, but right now there are other stories I'm more connected to. If it wasn't that story, I would've done nothing at all.
Then, two years later, I decided that if I wanted to make another webcomic, I had to be extremely attached and dedicated to it. Otherwise, it wouldn't make it that far. I still knew almost nothing about making webcomics, writing them, even drawing in general. But I learned so damn much from it. I made over 100 pages and don't regret a second of it. (Unfortunately, a bunch of bad things in my life caused me to be unable to work on it for a very long time, and I eventually decided I wanted to start something new and better.) There were an unbelievable amount of great ideas, characters, storylines, in that webcomic. Even despite not knowing how to do anything, how could I not choose that story to work on?
If you want to get a feel for how things work, I recommend doing a bunch of "test" pages. Maybe draw your characters doing or talking about something random. You could draw some part of a backstory or a scene you're really excited to show (and then just redraw it later). I personally decided to draw something silly when I was testing out formats for my current webcomic.

It was fun, and helped me see what I needed to change in terms of looks and speeding up my drawing speed.
As for the plot-- OUTLINE!! Make an outline, talk it out with someone (if you don't have anyone there are also servers & communities for these kinds of things or you can just dm me, im happy to help), and just figure it all out BEFORE you start drawing your main comic. Also, I would recommend scripting out the scenes then storyboarding them then doing your actual drawing.