For this you can use rendered attribute in your VF Page

say something like this

<apex:commandbutton value="Sign-Up" action="{!signup}" rendered="{!stagename=='ClosedWon'}"/>

Then this command button will appear only when the stagename is ClosedWon

You should be able to use a solution similar to this:

http://boards.developerforce.com/t5/Visualforce-Development/Display-command-button-when-any-picklist-value-is-selected/m-p/267501#M34443

But in your case you want to use the disabled attribute on the command button rather than the rendered option.

Although, keeping the button hidden rather than disabled would work as well and may be a better fit for what you're trying to do.

Hi Thanks for your reply.

But i have using java script Please look on my page

<apex:page controller="CustomerSetupFormeditpagectrl" tabStyle="opportunity">
<apex:form >
<apex:includeScript value="{!URLFOR($Resource.Jqueryui18, 'js/jquery-1.7.1.min.js')}" />
<apex:includeScript value="{!URLFOR($Resource.Jqueryui18, 'js/jquery-ui-1.8.18.custom.min.js')}" />
<apex:stylesheet value="{!URLFOR($Resource.Jqueryui18,'css/smoothness/jquery-ui-1.8.18.custom.css')}"/>
<script type="text/javascript">
var $ = jQuery.noConflict();
$(document).ready(function()
{
//alert('gdhgh');
$("input[id$=':cbtn']").attr("disabled", "disabled");
});
function checkEnableSubmit(value)
{
//alert('wwwwwwww');
if (value=='6. Started Trading - Closed Won')
{
//alert('ddddddd');
$("input[id$=':cbtn']").removeAttr("disabled");
$("input[id$=':cbtn']").css("color","Black");
$("input[id$=':cbtn']").css("Cursor","pointer");
}
else
{
$("input[id$=':cbtn']").attr("disabled", "disabled");
$("input[id$=':cbtn']").css("color","gray");
}


}
</script>
<!----<script type="text/javascript">

document.getElementById("cbtn").disabled = true;

function checkEnableSubmit(value)
{
// var str=document.getElementById("pickval").value;
//var str=document.getElementById('{!$Component.pickval}').value;
//var ex = document.getElementById('{!$Component.pickval}').value;
//var ele=document.getElementById('{!$Component.pickval}');

alert('rrr'+value);
if (value=='6. Started Trading - Closed Won')
{
alert('eeeee'+value);

document.getElementById('{!$Component.cbtn}').style.display="block";
}
else
{
document.getElementById('{!$Component.cbtn}').style.display="none";
}
}

</script> ----->

<apex:pagemessages ></apex:pagemessages>
<apex:sectionHeader subtitle="{!opp.name}" title="Opportunity Edit" />
<apex:pageBlock title="Opportunity Edit" mode="edit">
<apex:pageBlockSection title="Opportunity Information" collapsible="false">
<apex:inputField value="{!opp.name}"/>
<apex:inputField value="{!opp.Amount}" required="true"/>
<apex:inputField value="{!opp.accountid}" required="true" /><apex:actionregion >
<table width="100%" ><tr><td width="8%"></td><td width="110px'"><apex:outputLabel ><b>Pipeline Stage</b></apex:outputLabel></td><td>
<apex:inputField value="{!opp.stagename}" id="pickval" onchange="checkEnableSubmit(this.value);" >
<apex:actionSupport event="onchange" action="{!OnStageChange}" reRender="probability" /> </apex:inputfield>
</td></tr></table></apex:actionregion>
<apex:inputField value="{!opp.ownerid}" />
<apex:inputField value="{!opp.closedate}"/>
<apex:inputField value="{!opp.Customer_Code__c}"/>
<apex:inputField value="{!opp.Probability}" id="probability">{!Testval}</apex:inputfield>
<apex:inputField value="{!opp.Accepted__c}"/>
<apex:inputField value="{!opp.Reason_Won_Lost__c}"/>
</apex:pageBlockSection>
<apex:pageBlockSection title="Additional Information">
<apex:inputField value="{!opp.Opportunity_type__c}" required="true"/>
<apex:inputField value="{!opp.Branch__c}" required="true"/>
<apex:inputField value="{!opp.Products__c}" required="true"/>
<apex:inputField value="{!opp.Industry__c}" required="true"/>
<apex:inputField value="{!opp.Rates_Types__c}" required="true"/>
<apex:inputField value="{!opp.Date_Entered_into_pipeline__c}" rendered="{!admin==true}"/>
<apex:outputField value="{!opp.Date_Entered_into_pipeline__c}" rendered="{!admin==false}"/>
<apex:inputField value="{!opp.LeadSource}" required="true"/>
<apex:inputField value="{!opp.Campaignid}"/>
<apex:inputField value="{!opp.Runs_List__c}" required="true" />
<apex:inputField value="{!opp.Expected_Start_Date__c}" rendered="{!admin==true}"/>
<apex:outputField value="{!opp.tests__Expected_Start_Date__c}" rendered="{!admin==false}"/>
</apex:pageBlockSection>
<apex:pageBlockSection title="Market Intelligence">
<apex:inputField value="{!opp.Competitor_A__c}" required="true"/>
<apex:inputField value="{!opp.Our_Strengths__c}" style="width:90%"/>
<apex:inputField value="{!opp.Competitor_B__c}"/>
<apex:inputField value="{!opp.Our_Weakness__c}" style="width:90%"/>
<apex:inputField value="{!opp.Competitor_C__c}"/>
</apex:pageBlockSection>
<apex:pageBlockButtons >
<apex:commandButton value="Save" action="{!save1}"/>
<apex:commandButton value="Save & New" action="{!Saveandnew}"/>
<apex:commandButton value="Cancel" action="{!cancel}" immediate="true"/>
<apex:commandButton id="cbtn" value="Customer Sign-Up Form" action="{!Signupform}" />
</apex:pageBlockButtons>
</apex:pageBlock>
<!--<INPUT TYPE="submit" VALUE="Select Refferal" id="cbtn"/>-->
</apex:form>
</apex:page>

Am using above(red color) java script for button disable based on stage name(picklist values) but first time page load

working but when select stage value is "Closed-won" button will not enable but in this situation only button will enable?

Can any one help me? 

Thank you