How do I prevent my website from being hacked?

HTML and CSS alone are not vulnerable to attacks since there's not data being submitted to your server.

As soon as you bring in a server side language, a database, and user information, you have to concern yourself more with security. Common ones that should be on your mind, if you get this far, include CSRF attacks, Session hijacking, SQL injection, XSS attacks.. Additionally, making passwords securely hashed is extremely important, never store them as plain text in a database. However, a lot of these attacks are easily preventable through their own ways. The problem is that novices leave easy-to-secure doors open to hackers. Thus, they're the primary targets of hackers. If you're dumb enough to allow SQL injection on a production server, you deserve to be hacked.

However, HTML and CSS are static files and can only really be downloaded. They have no interactive features, and thus have no real security features. Even if they did, you wouldn't have any user data being vulnerable, so the risk is minimal. So if you're planning on sticking with just those, you don't have much to worry about.

The only vulnerabilities you could possibly have are at the server level... Like if you allowed SSH, FTP, or any other direct access to your server. In that case, a hacker could go on a rampage with your files and/or account.

Your Host has measures in place to prevent hacking.

I have a web page also and I don't have any thing special in the HTML/CSS.

I leave that up to my host.