This guide will explain how to use other services to create a web site that anyone on the Internet can browse. In fact, this page was originally set up using the exact instructions described here.
You should have completed at least the Intro to HTML/CSS: Making webpages lessons on Khan Academy, or completed similar lessons elsewhere. This is the absolute minimum level of knowledge required.
These optional lessons will also be useful to you, but it is not necessary to complete them before continuing here:
If you still just want to experiment and you want a less restrictive environment to do so, there's an excellent free service called Glitch that you can try. This provides you with an online editor, but your projects can consist of multiple files (including images) and you are less restricted in what you can do.
So if you've at least finished the Intro to HTML/CSS class on Khan Academy, keep reading.
Dropbox will give you 2GB of cloud storage for free, and KISSr will let you set up one site that allows limited page views per month for free. This is plenty to get started with.
First of all, if you're 13 or younger then make sure to ask your parents for some advice as to what you should or shouldn't put on your web site. This is one of the things that parents are for.
In general, avoid putting any sort of contact information on your web site. If you put your email address on your web site then you're guaranteed to get more spam, at the very least, and you will also likely get people sending you emails trying to scam you into sending them money or whatever. It should be obvious but never put your phone number or address on your web site! That is a really bad idea.
You probably also don't want to put any social media account links on your web site either. This also causes spam and you may get strange people sending you friend requests. It's pretty much the same thing as putting your email address on your web site; you get spam, scams, and similar when you do this. So just avoid any sort of contact information.
Also I'd strongly recommend following the advice from McGruff Safety Tips for Kids. Most of that is good advice for adults too.