In this excellent video the folks at Phlearn walk you through the process of using photoshop specifically for the purpose of making your photos look great on the web.
The good part starts at ~4m55s in this video...
The main learnings here are to do your sizing first and then sharpen after and to always duplicate your photos before resizing. Never work on the originals...
Image Apply Image (merged layer)
Right click on the new layer
Image, Image size (display size)
Sharpen (at display size)
Filter, Sharpen, Unsharp Mask (before export right at the end)
Radius (start at zero up arrow until it looks bad)
Same for Amount
Now save it out
File, export, save for web
CONVERT TO SRGB (key! this is the color space that browsers use)
Thats all you gotta do!