how to set Button enable and disable mode

Hello Uday

****Controller****

public class sampleCon {
public String selectedImage{get;set;}

public sampleCon(){
selectedImage = '';
}
public PageReference test() {
return null;
}

public List<SelectOption> getItems() {
List<SelectOption> Images = new List<SelectOption>();
Images.add(new SelectOption('hyd','hyd'));
Images.add(new SelectOption('che','che'));
Images.add(new SelectOption('bng','bng'));
return Images;
}

public String getImage() {
return selectedImage;
}

public void setImage(String selectedImage) {
this.selectedImage = selectedImage;
}
}

****VisualForcePage****

<apex:page controller="sampleCon">
<apex:form>
<apex:selectList value="{!selectedImage}" multiselect="true">
<apex:selectOptions value="{!Items}"/>
</apex:selectList><p/>
<apex:commandButton value="Test" action="{!test}" rerender="out" status="status"/>
</apex:form>
<apex:outputPanel id="out">
<apex:actionstatus id="status" startText="testing...">
<apex:facet name="stop">
<apex:form>
<apex:commandButton value="Submit" disabled="{!If(selectedImage == '',True,False)}"/>
</apex:form>
</apex:facet>
</apex:actionstatus>
</apex:outputPanel>

</apex:page>

Hello Uday

****In Controller****

This is our getter and setter method(getImage and setImage) ,Here we First get value of our Image. Then in setter method we set this value to our declared variable(String selectedImage).

****InVF Page****

There is an attribute disabled in "button" through this we enable or disable our button according o given condition.Condition is defined in If Block inside the disabled attribute.

If selectedImage is empty then button is disable,Otherwise it is enable.