I need a programme for my mom's business inventory. I tried to download Dev-C and Visual Basic so I could make one, but after downloading it, the software would not open completely in the computer.

well ... this has pretty much ... nothing to do with java ...
do you have any experience in programming? and what do you mean "would not open completely in the computer"??

since you are posting this in the java forum, are you considering writing it in java? the basics of what you need to write your application are:

the JDK (Java Development Kit) which can be found here
the command prompt

and the knowledge of how to use the above three.

It sounds like you don't have much programming experience? If that's the case then to make a real-life application like this from scratch is far too difficult a project for a beginner. Why not look at one of the free inveytpory applications that you can simply download and start using? Even if they turn out to be not-quite-right at least you will learn a lot about what you really need.
Have a look at things like http://www.top5freeware.com/inventory-management-software-for-windows