Where to start with learning how to use API's?

Hello. I'm a web developer, in training I guess lol, thats teaching himself how to build websites. So far I got pretty good at building nice websites using purely HTML and CSS and a little bit of JavaScript for some cool effects. I mainly use bootstrap as I find it super clean and learned a bunch from looking at free templates and stuff.

Anyway now I want to try to take it a step further. I want to dabble into API's. I watched a few videos and understand what they are and I feel like it's an important skill to know how to use them.

So my first "Project" I gave myself was to use the API of Amazon and Walmart and simply build a page where I type in let's say "iPhone 7" and it goes and searches it and get's the most relevant result from each respective store and shows it on my page with it's price, picture and what not so I can see where I can find it for cheaper rather than having to open two tabs and search on both sites.

But first I just wanted to simply create a page where I type in a products and it displays the most relevant product in Amazon.

So I read the documentation and got the access keys and such to make requests, and well.. that's as far as I got. I read the code and understood the PHP but I'm just not sure how I can put it all together and have it show up nicely.

If anyone could point me in the right direction towards learning how to do these sorts of things it would be greatly appreciated!