Every once in a while, you will find that you need to edit, add, or delete pieces of code on a website. More often than not, this will be HTML and CSS, though you may see some JavaScript or other languages embedded inside HTML. Whatever the case, the best website to go to is w3schools.com. This site has several resources for understanding several coding languages, as well as their proper style guides, and is suggested in many classes here at BYU, including IS 201 where you learn HTML/CSS.
Besides w3schools.com, you can look into Codecademy, Khan Academy, Code.org, or just Google it.