I would firstly, suggest you reading some online struts tutorial.
But, just to provide to with some insight; struts has a validation framework which gives to options of having two types of Validation:- 1. Client Side validation this can be done by using Validation Framework configuration using validation XML configuration file. This help in generating Client Side Java Script validation Code. This is good in most of common scenarios. But if you have a complex validation then i would suggest using the second option.
2. Server Side Validation this can be by extending ValidatorForm instead of Simple Action Form. you have implement Validate method in your form class also you have validation done in your action class.