if i get a second job is it illegal for my first employer to prevent me from going to my second job?

There are not likely any criminal penalties that would prohibit you from seeking a second job. Some employers, do however, prohibit their employees from taking on a second job. They may have a "no moonlighting" provision in their handbook. In addition, some employers will prohibit employees from engaging in employment that competes with their own or that is somehow in conflict with their own. You should review your employee handbook to see if there are any moonlighting, noncompetition provisions or conflict of interest provisions that speak to this issue. If there are none and there is no other language prohibiting you from taking on you are probably ok to pursue the other employment.