The error you mentioned is the result of having JRE 7 installed and trying to run a Forms 10 application with it. Forms 10 is not supported with JRE 7. Further, Forms 10 is no longer supported. You should consider using Forms 188.8.131.52. This is especially true if you are just trying to learn the product as it makes little sense to learn with an obsolete version. If you want to continue using Forms 10, you will need to uninstall JRE 7, apply the 10.1.2.3 patch, then install and use JRE 6.
If you are forced to stick with Oracle Forms 10 you can run your Forms Application outside your browser with tools like Oracle Forms EXEJar which allows you to run Forms Applications outside the browser (converts it into an executable AJR file) and without the need for jinit and the compatibility issues jinit comes with. I'll go and look for some links and post them here later.
If you are forced to stick with Forms 10 and you are using JRE 7, then you can solve the problem bei setting java parameter -Djava.vendor="Sun Microsystems Inc.", depending on your java version either in the Java Control Panel or in the environment variable JAVA_TOOL_OPTIONS.
see for example Java 1.6 and 1.7 on same computer