public class MyCustomController {
public Merchandise__C merchandise { get; set; }

public MyCustomController() {
        Id id = ApexPages.currentPage().getParameters().get('id');
        merchandise= (id == null) ? new Merchandise__C() :
            [SELECT Name, Description__C, Price__C,Total_Inventory__C FROM Merchandise__C WHERE Id = :id];
    }

public PageReference save() {
    try{
        insert merchandise; //Database.insert
        return (new ApexPages.StandardController(Merchandise)).view();
        }catch(Exception ex){
          System.debug('Exception : ' + ex);
        }
     return null;
    }

}

<apex:page controller="MyCustomController" tabstyle="Merchandise__c">
    <apex:form >
        <apex:pageBlock mode="edit">
            <apex:pageMessages />
            <apex:pageBlockSection >
                <apex:inputField value="{!Merchandise.name}"/>
                <apex:inputField value="{!Merchandise.Description__c}"/>
                <apex:inputField value="{!Merchandise.Price__c}"/>
                <apex:inputField value="{!Merchandise.Total_Inventory__c}"/>
            </apex:pageBlockSection>
            <apex:pageBlockButtons location="bottom">
                <apex:commandButton value="Save" action="{!save}"/>
            </apex:pageBlockButtons>
        </apex:pageBlock>
    </apex:form>
</apex:page>