It's really great to know web coding, and HTML and CSS are the basics, and best place to start, in addition JavaScript would be great to learn, you can do a lot with it. Just for the beginning stick to HTML and CSS, then jump to JavaScript to start learning more programmings.
A personal favorite source; http://www.codecademy.com/ ( Here you learn to code, by coding )
Another way I always recommend is; get some simple template you can find for free on the web, and tweak it, play with it, use the "Inspect element" in browsers to see how all rules affect the elements in the page etc.
Like LAJW said, anytime you have a question, head over to "http://www.w3schools.com/", they have everything with live examples, For more (of course you will need more help), just search on the web, sites like "https://css-tricks.com/" will save you a lot of time.
Just start and learn something everyday, and you'll bee all cool, very soon.
And most importantly, from the beginning use Percentage (%), and ems etc for measurements and completely avoid pixels(px), you can do almost everything.
Tips:
How to convert width I see on my screen, in pixels, to ems so it looks same on different resolution displays?
1 em = Font size set for the browser or device
If you want to convert 7 pixels to em; 7 / 14 = 0.5em ( When you browser's font size is set to 14px )
20 pixels to em; 20 / 14 = 1.428571428571429em
Some browsers will notice only up to some decimal points but some will take into account more, so if you will just copy and paste the number from your calculator and not round it up, it will help you with precision.
When you need to convert a very small width, say 1px for border or something, instead of just converting 1 pixel to em, convert 1.25 pixels (for example) to em; 1.25 / 14 = 0.0892857142857143em
It helps with browser compatibility.
Best wishes!
Stay safe!