Are open ports dangerous?

I have forwarded port 80 to WAMPServer and 25565 to the MineCraft-server running Bukkit. (http://www.bukkit.org/)

I assume that you're talking about forwarding those ports from a router / firewall / NAT box to your own machine rather than forwarding them to some off-site instance. Usually we call this "opening" a port.

Am I vulnerable to attacks. Can hackers attack me if WAMP-server is running or if it is not running, or will it be easier to hack me than if I hadn't opened the port at all?

Yes, you are more vulnerable by running a service than by not running a service. Adding more services increases attack surface. That doesn't mean by that you're vulnerable by definition, but an increased attack surface increases is one of the factors that increases your risk.

I have tried to learn about how people hack and how to protect myself from it. Do you know where I can read about it?

The more time I spend here, the greater resource I find this site to be. I've never found a really good comprehensive guide to how to be secure. There are books like the Hacking Exposed series that talk about a lot of technical aspects to security and some of the mindset. As much as the CISSP might be railed upon, trying to study up for that exposes one to most of the different knowledge domains that a professional should be aware of. I think that reading questions on this site is a great way to learn.

The challenge is that security is a mindset. The technology and even some of the fundamentals of it are an evolving realm. You learn by exposing yourself through trial and error. It is helpful to make as much of that exposure as possible vicarious -- learn from others' mistakes.

Security as an equation works like this:

• Decide what you need to offer -- you have a need to provide something with a WAMP server and Minecraft game server.
• Limit everything you don't need to offer and do it in depth.

When I say in depth, I mean that only offering those services is a start. Try to make sure that the programming code on your website is written securely. Then, pretend it has been compromised and consider what can happen from there. Is the Apache process prevented by the operating system controls from writing files anywhere? Do you have a method to detect compromise? Keep asking those questions and finding layers of answers so that if you are hacked, you mitigate your risk.