1. I'd also recommend Visual C++ Express or Code:Blocks. I'd never heard that you couldn't make closed source software with Dev-C++, but it isn't usually the better choice to use regardless.
2. Make something worth selling then, and learn marketing, which would be your new best friend.
3. I'm not going to list everything here, but in most cases of open source software you can generally use them without limits, as far as the product. The obligations under some licenses applies to the program itself, for example with Blender(for 3d modeling) if you make source code changes, they want you to release those to the community. And you can't claim the original program was all your own creation...but any models, textures, animations, etc... that you make from it belong to you and you alone, with no credit required or anything. GIMP I believe is similar for 2d graphics editing, as are many open source programs. You can also check http://www.gnu.org/licenses/ to see the various licenses and what you can do with related software. Also, any website that has said software will usually not only have the license, but in the FAQs or somewhere similar they explain in layman's terms what you can and can't do.
4. The GPL license(for code) forces you to make your project also GPL, which you may not want. It doesn't say you can't commercialize your project, but you'd have to open the source, which would also include images, etc... The LGPL is basically the same, except that you are allowed to externally link to the code and not have to GPL your project. If you statically link to the code(which makes the code directly part of your executable) then the GPL applies, but if you dynamically link the code(like calling functions from a DLL) then you DON'T have to make your project GPL.
5. There are many pieces of software that are LGPL or even other licenses, like BSD, which generally are more free to use, with less requirements. The main point of those "free" licenses is simply to allow you to use the software in any way you want, except to take credit for the original software. Lastly, there are some licenses that allow any usage whatsoever with credit. This applies to a lot of music and sound effects you can find, but it can apply to software.
If you post or PM me exactly what kind of project you are doing, I may be able to recommend more specifics on APIs/libraries to use, but remember that you can also look right here on GameDev to find lists of different types of software with license information, and devmaster.net has a massive list of engines, commercial/open-source/etc.. that you can use to learn about the many options.
