First, realize that web dev and web design are two different (although connected) fields.

You should have very little trouble with web dev, based on your coding and database background.

To get better at design, read and practice! Alistapart.com, csszengarden.com, etc., are all great places to find inspiration. 37 signals is another.

Finally, don't spend too much thought on the "scalable/optimized back-end". The only time you'll ever need to worry about that is if you're successful, which will depend primarily on your idea, execution, speed, etc., and much less on if your system scales.

So to sum: read design journals. Look for inspiration (include real life). Practice your basic frontend skills. Don't waste brain cycles optimizing.