Try this code snippet and change according to you:
<apex:pageBlockSection title=”Select Countries”>
<apex:inputCheckbox value=”{!reUSA}” label=”USA”><apex:actionSupport event=”onchange” reRender=”USASectionOuter” id=”Us”/></apex:inputCheckbox>
</apex:pageBlockSection>
<apex:outputPanel id=”USASectionOuter”>
<apex:outputPanel id=”USASection” rendered=”{!reUSA}”> <apex:inputField value=”{!abc__c.myfield__c}” /></apex:outputPanel>
</apex:outputPanel>
Thanks.