If you use CodePen you just write HTML, CSS and JS in respective windows and it will be combined automagically (good in the beginning, but I recommend to switch to some external editor as soon as you feel comfortable).
If you use additional libraries (Bootstrap, jQuery etc.) you add them in Codepen Settings.
You try to break the project in smaller pieces and then google, google and google again.
I usually start with basic HTML/CSS skeleton, just to have elements to attach JS to.
Then I think what I need to accomplish and if I have no idea how to do it I just google in plain English.
For example “get random quotes from internet”. Click through first couple of results (I finally used this). Now I know that I need to use API, JSON and callback. Try example API requests (paste in browser http://api.icndb.com/jokes/random), see what I get. Read on MDN about JSON, callback etc.
I restarted my Random Quote Generator three times (completely deleted the code and started from scratch) before I got more or less satisfactory result (although now I look at my code and it looks like crap to me).
P.S. Some campers do this project using a fixed array of quotes, but I think the goal of this project is to learn to use APIs.