Sangan - When no other process action can get the job done, add customized functionality to your Salesforce processes by calling an Apexmethod.To call an Apex method, add the Call Apex action to your process and select an Apex class with an invocable method. It’s that easy.If the class contains one or more invocable variables, manually enter values or reference field values from a related record. Each value must match the variable’s data type.
Here's an example to get you started , call apex class from process builder (http://salesforceworld4u.blogspot.in/2015/05/how-to-call-apex-class-from-process.html)
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You have to use "Apex Code " action from process builder. As mentioned in below screen shot.
Thanks,
Anurag
Just follow the steps given in https://automationchampion.com/2015/07/09/getting-started-with-process-builder-part-33-call-an-apex-method-from-a-process/