If you can't come up with ideas on your own, you will not be able to write a comic. Not one worth reading, anyway. Writing comics (or any type of story) requires a constant stream of ideas, as well as the ability to weed out both good and bad dieas that don't improve the story (or comic as a whole, if it's a gag-type comic with no story).
There are, however, creativity exercises you can do. Try using the generators at
Seventh Sanctum and flesh out the things it gives you into stories. This probably won't give you a story worth making into a comic, but it'll help you learn to connect unconnected ideas - the basis for creativity.
As for cartoon styles - it's all about simplification and exaggeration. Understand what you're simplifying and what features make it what it is, and exaggerate them. Cartooning arguably requires a deeper understanding of things than merely rendering them realistically, despite the seemingly simpler outcome.
Depending on the comic's tone/plot, you might go for a more or less exaggerated/simplified style. Don't forget to think about that.
And don't forget about things like composition and panelling, they're as important as anything else in a comic, but are frequently overlooked. There are a million threads on these things already, so I won't repeat my usual spiel here. Just look around.