I'm really crap at drawing and would like to know if there is a certain app/website that you use to visualise characters that you can create.
Only thing that comes to mind is heroforge, which is for designing custom miniatures. It's more geared towards clothing, armor, weapons and items, and as such doesn't have a whole lot of facial customization. Might be worth looking into regardless.
i usually use a free photo site like pixabay , plus photos from the net to 'cast' a character rather than trying to draw them
A lot of the time, I just do specific image searches. I also design them in Blender 3D. They're all works in progress, however, since I'm basically a low-poly designer. It's tricky getting characters with less than 2000 polygons looking good. I hope to one day have a computer that can handle millions of polys in a scene.
Sometimes I use pinterest, or a more general search. On rare occasion I do try and draw them, but I'm not the best.
How do you see yourself? Try to put yourself in your character's shoes and try to see themselves in their perspective or, even better, people around them. Average novels and stories nowadays are written through inside-out perspective rather than outside-in view. Every perspective is, for the lack of better words, askew. Else socialisation would be such a bore and a chore, and no narrative can be written or even told.
I usually use a Pinterest board or just generally search for stock image models and save them in a collection folder on my MS browser. To be honest I even used Sims 3 when I was younger and wrote short modern stories or FanFic. I drew one character I used in role playing games, and could technically do it for others but I feel like if you spend too much time trying to create a perfect image of your characters, you're going to throw away a lot of time you could use to actually write.
I generally find that a character's exact visual appearance matters very little in stories, unless you're writing a graphic novel.
As someone that has aphantasia I can't visualise anything in my mind at all, so what I tend to do is use Pinterest/Google searches to find images of models/actors/etc that fit the look I am going for and then use them as reference. I don't stick to them to a T, but it certainly helps me remain consistent and have a base to start from.