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I've used The bootstrap framework to make web apps that don't look horribly ugly. Learning all the things you'd need to make apps that use that (so a bit of JS, CSS, HTML, etc. as sixes_and_seven says) would probably be a good start. (It would be probably easier than trying to make good-looking CSS from scratch, which is more of a pain).
Bootstrap is particularly good if you're a design doofus and have minimal knowledge of web standards, accessibility, fluid layouts, etc.
I'm sure ancestor commenters know this, but it's worth mentioning that design is a distinct discipline which doesn't come for free when you learn to code.