At minimum, a custom controller that uses the Apex Messaging namespace needs a subject, a body, and a recipient for the email. You will need a page that acts as a form to fill out the subject and body and deliver the email.
<apex:page controller="sendEmail"> <apex:messages /> <apex:pageBlock title="Send an Email to Your {!account.name} Representatives"> <p>Fill out the fields below to test how you might send an email to a user.</p> <br /> <apex:dataTable value="{!account.Contacts}" var="contact" border="1"> <apex:column > <apex:facet name="header">Name</apex:facet> {!contact.Name} </apex:column> <apex:column > <apex:facet name="header">Email</apex:facet> {!contact.Email} </apex:column> </apex:dataTable> <apex:form > <br /><br /> <apex:outputLabel value="Subject" for="Subject"/>:<br /> <apex:inputText value="{!subject}" id="Subject" maxlength="80"/> <br /><br /> <apex:outputLabel value="Body" for="Body"/>:<br /> <apex:inputTextarea value="{!body}" id="Body" rows="10" cols="80"/> <br /><br /><br /> <apex:commandButton value="Send Email" action="{!send}" /> </apex:form> </apex:pageBlock> </apex:page>
https://Salesforce_instance/apex/sendEmailPage?id=001D000000IRt53
public class sendEmail { public String subject { get; set; } public String body { get; set; } private final Account account; // Create a constructor that populates the Account object public sendEmail() { account = [select Name, (SELECT Contact.Name, Contact.Email FROM Account.Contacts) from Account where id = :ApexPages.currentPage().getParameters().get('id')]; } public Account getAccount() { return account; } public PageReference send() { // Define the email Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage(); String addresses; if (account.Contacts[0].Email != null) { addresses = account.Contacts[0].Email; // Loop through the whole list of contacts and their emails for (Integer i = 1; i < account.Contacts.size(); i++) { if (account.Contacts[i].Email != null) { addresses += ':' + account.Contacts[i].Email; } } } String[] toAddresses = addresses.split(':', 0); // Sets the paramaters of the email email.setSubject( subject ); email.setToAddresses( toAddresses ); email.setPlainTextBody( body ); // Sends the email Messaging.SendEmailResult [] r = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email}); return null; } }