The UI looks pretty dated. Feels like something I would have seen in Windows XP.
For starters, learn color theory and study some schematics. I like that you're staying with one hue (blue) so keep that style, but choose different shades that blend seamlessly (you'll know once you study).
Modern UI designs utilize white space for a sleek and professional look. Perhaps shrink the font sizes a tiny bit along with the various UI elements.
This design feels rather cartoonish for an eCommerce application. Maybe sharper edges on the icons and little designs should help mature it a bit.
As for the font, I recommend looking into free application fonts (Sans-Serif fonts for the header and Serif fonts for everything else). If you want the paid fonts, I recommend Helvetica Neue, pretty much the standard app font. The font you chose currently looks like the font from Cat Mario.
For now, I'd say you should study UI design a bit to understand how to properly make one, but I see you already have the basic framework down which is good. Great effort