Let me take a brief moment and give you a warning about copyrights. When you are making games and acquiring content for those games, it is extremely important that you pay attention to copyrights. If you don't, it will come back to haunt you.
Rule #1: Assume everything is copyrighted.
In most countries, including the United States, you don't have to actually file a copyright claim or anything, in order to get a copyright. The simple fact that a person creates it means they have a copyright on it.
Copyrights work on just about everything you'll put into a game, including, but not limited to fonts, pictures, textures, 3D models, music, sound effects, and source code.
Source code can be a real kicker. That means that any code libraries you use, or even code snippets you find, both on this site, and others, have a copyright on it.
If you can't find any copyright information, DO NOT assume that this means that it is free to use. Email the creator for permission. And it might be a good idea to save that email.
Rule #2: It is dangerous to use anything that is copyrighted, without a license from the creator.
A copyright means that the person or company that made it gets to decide how the stuff is to be used. This is typically done through a license agreement. Agreeing to a license does not give you copyright. The original owner still has it, but you are allowed to use it for anything that they say you can.
Rule #3: Not all license agreements allow you to use stuff commercially.
If you are trying to make a game that you want to sell for profit, pay careful attention to what the license says. Many licenses will allow you to use their stuff for educational, or non-profit purposes, but ban you from using it commercially.
Free stuff works this way all the time, so be especially weary of free resources and code libraries.
Let me make one point very clear: the fonts, images, and other resources that come, as a part of your operating system, or any other application you have installed are not necessarily free for you to put into your game. Some of them are, but without understanding the copyright on them, you could get yourself into trouble, once you go to sell your game.
Rule #4: Be especially careful with share-alike clauses, that are common in open source licenses.
Many open source licenses (the GPL, for example) state that you can do anything you want with it, including sell it commercially, but you must "share alike", meaning that you must also license your work under the same license.
While there is still a lot of debate about this, many of these "share alike" licenses may technically prevent you from actually selling your game commercially. If you are required to put your work under the same license, you could sell it to someone, but since they receive it under a "share-alike" license, they can do anything they want with it themselves, including giving it away for free, or selling it themselves, thus preventing you from actually being able to sell it.
One more important point: I'm not saying that open source licenses are bad. They're not. They've got an important role in today's computer world. But open source, and proprietary, commercial software are archenemies. It is extremely difficult to get them to cooperate with each other.
Rule #5: For the most part, the code examples and resources you find in these tutorials are fair game.
Any of the stuff I put in these tutorials, whether it is code, fonts, images, 3D models, and so on, are free for you to use, both during the learning process, and also for commercial uses if you want. You don't need to give me credit for them or anything. Just use it.