Creating Pages for Object-Specific Custom Actions:-

The following code sample shows a page designed to be used as a custom action on the account object, so it uses the standard Account controller. This action lets users create cases from account detail pages, and it has a different user interface from standard create actions.

public with sharing class CreateCaseExtension {
    private final SObject parent;
    public Case theCase {get; set;}
    public String lastError {get; set;}
    
    public CreateCaseExtension2(ApexPages.StandardController controller) {
        parent = controller.getRecord();
        theCase = new Case();
        theCase.accountId = parent.id;
        lastError = '';
    }
      
    public PageReference createCase() {
        createNewCase();
        theCase = new Case();
        theCase.accountId = parent.id;
        return null;
    }
       
     private void createNewCase() {      
        try {
            insert theCase;
            
            FeedItem post = new FeedItem();
            post.ParentId = ApexPages.currentPage().getParameters().get('id');
            post.Body = 'created a case';
            post.type = 'LinkPost'; 
            post.LinkUrl = '/' + theCase.id;
            post.Title = theCase.Subject;
            insert post;
        } catch(System.Exception ex){
           lastError = ex.getMessage();
        }
    }   
}

<apex:page standardcontroller="Account" extensions="CreateCaseExtension" showHeader="false">
    <script type='text/javascript' src='/canvas/sdk/js/publisher.js'/> 
    <style>
        .requiredInput .requiredBlock, .requiredBlock {background-color: white;} 
        .custompubblock div {display: inline-block;} 
        .custompublabel {width:54px;} 
    </style>
    <script> 
        function refreshFeed() { 
            Sfdc.canvas.publisher.publish({name : 'publisher.refresh', payload : {feed:true}}); 
        }
    </script>   
    <div> 
        <apex:form > 
            <apex:actionFunction action="{!createCase}" name="createCase" rerender="out" 
            oncomplete="refreshFeed();"/> 
            <apex:outputPanel id="out" > 
                <div class="custompubblock"> 
                    <div class="custompublabel">Account:</div><apex:inputField value="{!theCase.accountId}" 
                    style="margin-left:0;"/>&nbsp;&nbsp;&nbsp;
                    <div>Contact:&nbsp;</div><apex:inputField value="{!theCase.contactId}" />
                </div>
                <apex:inputField value="{!theCase.description}" style="width:538px;height:92px;margin-top:4px;" />
                <div class="custompubblock" style="margin-top:5px;"> 
                    <div>Status:&nbsp;</div><apex:inputField value="{!theCase.status}" />&nbsp;&nbsp;&nbsp; 
                    <div>Priority:&nbsp;</div><apex:inputField value="{!theCase.priority}" />&nbsp;&nbsp;&nbsp; 
                    <div>Case Origin:&nbsp;</div><apex:inputField value="{!theCase.origin}" /> 
                </div> 
            </apex:outputPanel>
        </apex:form><br/>
        <button type="button" onclick="createCase();" 
        style="position:fixed;bottom:0px;right:0px;padding:5px 10px; 
        font-size:13px; font-weight:bold; line-height: 
        18px;background-color:#0271BF;background-image:-moz-linear-gradient(#2DADDC, #0271BF);background-repeat:repeat-x;border-color:#096EB3;" 
        id="addcasebutton">Create Case</button> 
    </div>  
</apex:page>

You can try another method 

Pages for Global Custom Actions:-


The following code sample shows a Visualforce page designed to be used as a custom action on any object that supports actions. This action lets users create cases from record detail pages, Chatter, Chatter groups (except customer groups), or the home page, and it has a different user interface from standard create actions. As with all global actions, the records created through this action are not automatically associated with any other records.

<!-- Custom controller -->
public with sharing class CreateCaseController {
    public Case theCase {get; set;}
    public String lastError {get; set;}
    public CreateCaseController() {
        theCase = new Case();
        lastError = '';
    }
      
    public PageReference createCase() {
        createNewCase();
        theCase = new Case();
        return null;
    }
       
     private void createNewCase() {      
        try {
            insert theCase;
            
            FeedItem post = new FeedItem();
            post.ParentId = ApexPages.currentPage().getParameters().get('id');
            post.Body = 'created a case';
            post.type = 'LinkPost'; 
            post.LinkUrl = '/' + theCase.id;
            post.Title = theCase.Subject;
            insert post;
        } catch(System.Exception ex){
           lastError = ex.getMessage();
        }
    }   
}

<apex:page controller="CreateCaseController" showHeader="false">
    <script type='text/javascript' src='/canvas/sdk/js/publisher.js'/>
    <style>
        .requiredInput .requiredBlock, .requiredBlock {background-color: white;}
        .custompubblock div {display: inline-block;}
        .custompublabel {width:54px;}
    </style>
   
     <script>
      function refreshFeed() {
          Sfdc.canvas.publisher.publish({name : 'publisher.refresh',  payload : {feed: true}});
      }
     </script>
   
    
    <div>
        <apex:form >
            <apex:actionFunction action="{!createCase}" name="createCase" rerender="out" oncomplete="refreshFeed();"/> 
            <apex:outputPanel id="out" >
                <div class="custompubblock">
                    <div>Subject:&nbsp;</div><apex:inputField value="{!theCase.subject}" style="width:500px;" />
                </div>
                <div class="custompubblock">
                    <div class="custompublabel">Account:</div><apex:inputField value="{!theCase.accountId}" 
                    style="margin-left:0;"/>&nbsp;&nbsp;&nbsp;
                    <div>Contact:&nbsp;</div><apex:inputField value="{!theCase.contactId}" />
                </div>
                <apex:inputField value="{!theCase.description}" style="width:500px;height:92px;margin-top:4px;" />
                <div class="custompubblock" style="margin-top:5px;">
                    <div>Status:&nbsp;</div><apex:inputField value="{!theCase.status}" />&nbsp;&nbsp;&nbsp;
                    <div>Priority:&nbsp;</div><apex:inputField value="{!theCase.priority}" />&nbsp;&nbsp;&nbsp;
                    <div>Case Origin:&nbsp;</div><apex:inputField value="{!theCase.origin}" />
               </div>
                <div style="color:red;">{!lastError}</div>
            </apex:outputPanel>
        </apex:form><br/>
        <button type="button" onclick="createCase();" style="position:fixed;bottom:0px;right:0px;padding:5px 10px; 
        font-size:13px; font-weight:bold; line-height: 
        18px;background-color:#0271BF;background-image:-moz-linear-gradient(#2DADDC, #0271BF);background-repeat:repeat-x;
        border-color:#096EB3;" id="addcasebutton">Create Case</button>
    </div>            
</apex:page>