I'm going to be building a simple blog for myself.

Partially I just want something really simple and customizable, and also it will be a fun little programming project.

I'll be using PHP and mySQL for the backend. I won't be using any sort of framework as it shouldn't be necessary for a very simple blog. I'm fairly comfortable with JavaScript.

What I'm imagining is some sort of JavaScript library I can just download, link to my html and then turn a textarea into a simple wysiwyg editor. It could be as simple as a markdown editor or something with a little more features.

It has to be free. Open source would be a plus.

If anyone has any recommendations or advice I would be very grateful. Thanks!

I think I've identified an unfulfilled need in our current online environment, have an idea of an end-result that could flourish, and some possible ways to monetize without being a cancer on society.
I have some experience with basic site-building, but this would be a very ambitious project for one person. It will probably take a long time before I get it going—if I get it going at all. I know some of what I need to start learning to make it work, but my mind doesn't know where to start when it comes to security, stability, or scalability.

Security:
This is probably my biggest concern. We hear all the time about data breaches, and this idea is—at its core—a kind of social media; by its nature we're dealing with PII.
Where does everyone go wrong here, and how do I protect the information of potential users?
All I've got is I know that storing passwords in plaintext is a bad idea, and to encrypt 'stuff' .

Stability:
I'm sure we've all experienced a site going down to a heavy load. Say I can get a site up, and things go well for a bit, and then somebody with a big twitter/instagram/youtube audience says something and a hoard of people come to check it out. A site going down when you try to check it out doesn't leave a positive impression. Can this sort of thing be prepared for, in the possibility such a thing ever happened?

Scalability:
If I can get a workable site running, I don't expect much of a userbase for a while, if ever. But if it does get large enough to attract hundreds of thousands, or a million+ returning users, it would be nice if I could have the architecture in place to handle that ahead of time, instead of playing catch-up later. What makes a site scalable, and is there any reason it can't be prepared for?

I'd love to be able to pay some folk to help me, but I don't have the cash for that. The initial launch will have to be done by me alone, so I'm fully prepared to learn what I need to and take the long amount of time required (maybe next summer?) to get the minimum product going. Even if this ends up not being feasible to get going alone, I figure the worst case scenario is a lot of learning along the way.