User u = new User();
Contact c= new Contact(AccountId = DEF_ACC_ID,....);
insert c;
u.ContactId = c.Id;
insert u;

<apex:page controller="MobileAppLoginCtrl" action="{!onInit}">
</apex:page>

public class MobileAppLoginCtrl {

    public PageReference onInit() {
        String un = ApexPages.currentPage().getParameters().get('un');
        String pass = ApexPages.currentPage().getParameters().get('pw');
        PageReference pr = Site.login(un, pass, '/one/one.app');
        pr.setRedirect(true);
        System.debug (pr);
        return pr;
    }
}