Well, as with anything, it requires practice. Though, the biggest skill every professional designer has is the ability to find things that need fixing or that just don't look right. Along with this you need a passion for your work, the want to take it to the last step and polish it. To list a few things that need fixing that should pop out to you (for the homepage only):
1.) The background doesn't tile properly.
2.) The main "hero" image looks stretched out. Always keep an image in correct proportions. You'll also want to take your time (again, have passion for the work) and properly cut her out. It looks like a color/background selection wand was used, giving a jaggy look. There are better methods out there for doing this, and although they take more time, it is worth it.
3.) Your side menu's background is blue, but the text is blue... For any navigation, contextual elements, or text you want to allow for almost the max contrast that is possible. For example, complete black or white for the side menu font color will provide better contrast, though the white font will give better aesthetic value.
4.) At least on my browser (firefox 3.6.3), the side menu text runs off past the background, towards the bottom of the page.
5.) The "featured free shipping shoes" text, also feels stretched, you always want to keep an image in its original proportions.
5.) The same text just mentioned should be centered in its section since you have the same paradigm throughout the rest of the page for heading text. Always be consistent with whatever method/design paradigm you choose.
6.) A lot of the images on the page are large, yet they've been scaled down by html (meaning you changed the resolution the image displays at, but not the image itself). This is bad for two reasons. First it inflates the image size, a resampled image of the same size could be half or less the file size. Secondly, not all browsers actually resample their scaled images (aka work at the pixel level to scale the image down) and instead may simply resize the image (throws out whole line of vertical and horizontal resolution), which can lead to the images looking terrible and inconsistent across browsers. Your worst offender is this image:
Its unneedly large for its size on the site, and its a .bmp, so its 540kb! A tad too large for a small thumbnail on a site.
7.) This is more of a personal choice, but if its an ecommerce site, your header is a tad too big (though looks nice). You always want to have a reduced header to move content up above the fold (the point where content goes off screen vertically, based on your most common resolution).
Now keep in mind, this is only the design aspects. I am not sure whats going on under the hood, so to speak. If you want to be a developer you'll need to know at least one server side language and the general structured query language (sql), to create dynamic pages and pages that use templates. Not sure what you're doing now, if you can give me details i can give you some pointers.