Using Sketch won't make you a web designer. Neither will Photoshop. ;)
I'm coming from a front end developer position (mostly templating, no apps), so I've become good at designing directly via code. I don't even touch Photoshop anymore. I just directly build my idea in HTML/CSS and a bit of JS.
Like @cutcodedown says: Get the basics right first. From there read up on the possibilities of CSS3 and media queries specifically. The biggest gamechanger for me personally was to design mobile first. It does wonders on how you think about and structure a page.
Other than that look around for inspiration on Dribbble, Webdesigner Depot etc. There's lots of inspirational sites/blogs out there.
I'm not sure if there's some learning path out there, like an online course or something... You get the most experience by just starting and doing and failing a lot. ;)
And most important of all: have fun with it!