If you decided you wanted to be a web dev, probably your path must have been similar to mine: first HTML5, then CSS and later JS.
But then a friend of a friend hears that you are now a web developer, and asks you to make a Wordpress for them.

As a developer, I found Wordpress limited, confusing and so restricted! Something I could easily solve by modifying a CSS file or just by changing a few tags in the HTML was so hard!

Where are the files? Why isn't there any index.html?

So, you had to face a Wordpress development when you thought it was easy, and everything looks like if you had absolutely no idea where to start from.

So, if this job is actually something punctual, and you do not think to focus your career in Wordpress development maybe these tricks could actually work for you:

Know what the client wants

Probably the hardest part, especially if they are not quite technical. But you should at least now what structure the client wants the web to have and of course the functionalities.
It always helps a lot to have the content of the website (texts and images). If the client doesn't have this, then it might be a bit more difficult. But you can always play with dummy content.

Start from a template

At least you are feeling brave, do not start from scratch. Wordpress works with php. Which is a language that mixes the HTML tags with the language itself and is not pretty similar to JavaScript.
Look for free (or paid if you consider) themes that have most of the requirements the client wanted: a blog, an about section, a single page design... you name it. Just save you time for later.
Do not limit yourself to the Wordpress browser. There are plenty of cool designs out there you can import to Wordpress.

Make a child-template

This is a must. Once you have your theme installed, install this plugin: Child Theme Configurator and make a child-theme.
This is the best practice when working with templates. It is easier to solve mistakes and your changes won't be overridden by a theme update.

Ask full access to the hosting site

You will need access to the hosting site, in order to access the Wordpress files and start working.

Use an FTP manager

Download and install an FTP manager such as WinSCP. Connect it to the hosting. You will need to ask for permission in the host site, but it will surely have a tutorial on how to connect it.

Copy the original theme to local

You need to set up your playground, so copy the original theme to local, and also your child-theme archives.
Modify anything in local, and once you are ready, upload it.

What now?

Ok, you are ready to start. The first thing you need to know is how Wordpress actually works. Here I suggest a cool article with a straight-forward explanation on this.

After this, go to the Wordpress Administration Tool and play around. Modify as much as possible with the Wordpress tool. If you find that something cannot be modified, and there are no plugins to fix that, then that is the time to jump into the code.

Using a Code editor with a powerful search engine such as Visual Studio Code, will improve your task of searching for the functions.

Resources

Wordpress Codex: here's where Wordpress says anything that can be done with their platform. You may need it if you want to get more data from other places.

May the force be with you!