I'd say start with setting a goal.
Like, what do you envision when you draw your main character(s)? Do you see them slaying dragons? Stealing treasure? Overthrowing governments?
Or maybe just planting gardens? Going to school? Cooking food? Start with the activities that resonate with them, and plan a story from there.
For example, if I see my characters as 'dragon slayers', I'll try to write a story where at least one of the goals is 'slay a dragon'.
How do we get to that point, though? We need to set other goals. Maybe the dragon lives in a faraway castle...so we add the goal 'travel to the castle'. Or maybe even 'find the castle'.
But why is the dragon living there? Perhaps something happened to the original inhabitants. If we want this as part of the present story, we might add 'have the dragon kill the people in the castle', or 'have the dragon move into the ruined castle' (and perhaps 'have the dragon find out how the castle fell'~).
If we want this as a backstory, we might add 'have the characters find out how the castle fell, and what the dragon has to do with it (if anything)'. This could be broken up into smaller goals sprinkled throughout the story...maybe there are side characters that will each give them some segment of this information, and subplots attached to each. The story is getting pretty big already. ^^
In the end, all you have to do is choose the goals you want and put them in order:
'have the characters find out how the castle fell'
'find the castle'
'slay the dragon'
^That's pretty much a basic storyline right there. You could leave it like this and create a comic with a very simple story (like a children's picture book, for example) or you can break up the goals and nest other goals within them to add complexity.
This may be an unpopular opinion, but yes. ^^; Deciding if a story is 'engaging' is extremely difficult if you haven't written it yet. Same for whether or not it makes sense.
There are plenty of amateur writers who get so obsessed with those worries that they get trapped in recursive loops of starting to write the story, trying to figure out whether something is wrong with it before they're even halfway done, becoming overwhelmed with anxiety, scrapping it, and starting over.
Obviously, it's not fun, and you don't want to be one of them. =[ And the saddest part is they learn very little about developing a story past the beginning stages (which is arguably the hardest part) because they never let themselves get that far.
So yeah. Start with writing what you want (and finishing your work) and THEN try to gauge how good it is and how you can improve. It'll also be easier to get feedback when you can hand people a complete story to judge, instead of just bits and pieces.