VfPafe:
==========================
<apex:page controller="AssignmentEmail" >
    <apex:form >
        <apex:pageblock title="Send Email">
            <apex:pagemessages id="Showmsg"  />
            <apex:pageBlockSection columns="1">
                <apex:inputText label="To Address:" value="{!toEmail}" />
                <apex:inputText label="Subject:" value="{!Subject}"  />
                <apex:inputTextarea richtext="true" rows="20" value="{!Body}"/>
            </apex:pageBlockSection>
            <apex:pageblockbuttons >
                <apex:commandButton value="Send Email" action="{!SendEmailMain}" rerender="Showmsg"/>
            </apex:pageblockbuttons>
        </apex:pageblock>
    </apex:form>
</apex:page>
=======================================
apex class:
==============================
public class AssignmentEmail 
{    
    public string toEmail{get;set;}   
    public string Subject{get;set;}
    public string Body{get;set;}
    Public Boolean RequiredFieldcheck;
    
    public AssignmentEmail()
    {
        RequiredFieldcheck=true;
    }    
    Public void SendEmailMain()
    {
        If(toEmail=='')
        {
            ApexPages.addmessage ( new ApexPages.Message(ApexPages.Severity.Error,'Enter the Email address'));
            RequiredFieldcheck=false;
        }
        if(RequiredFieldcheck==true)
        {    
            SendEmail() ;  
        }
    }  
    
    public void SendEmail()
    {
        Messaging.SingleEmailMessage email= new Messaging.SingleEmailMessage(); 
        email.setSubject(Subject);
        email.setPlainTextBody(Body);
        List <string> Address = new List <string>();
        Address.add(toEmail);
        email.setToAddresses(Address);
        //Messaging.sendEmail(new messaging.SingleEmailMessage[] {email});
        List <messaging.SingleEmailMessage> mail = new List <messaging.SingleEmailMessage>();
        mail.add(email);
        messaging.sendEmail(mail);
        ApexPages.addmessage(new ApexPages.message(ApexPages.severity.CONFIRM,'Email Sent successfully!'+' '+toEmail+ ': '+Address +' '+'With Subject : '+Subject));
    }   
}