Hi Laxmi, 
You can implement this requirement with Process builder and trigger as well.  Please find the Sample code for trigger here. But make sure you need to follow the best practices that followed by the community.
 

trigger SendEMailOpportunity (before update) {
Map<Id,Opportunity> oldMap = trigger.oldMap ; 

for (Opportunity opp : Trigger.new) {

Opportunity  oldOpp = oldMap.get(opp.Id) ;
if(oldMap.IsChecked__c==false && opp.IsChecked__c==true){
// First, reserve email capacity for the current Apex transaction to ensure
// that we won't exceed our daily email limits when sending email after
// the current transaction is committed.
Messaging.reserveSingleEmailCapacity(2);

// Processes and actions involved in the Apex transaction occur next,
// which conclude with sending a single email.

// Now create a new single email message object
// that will send out a single email to the addresses in the To, CC & BCC list.
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();

// Strings to hold the email addresses to which you are sending the email.
String[] toAddresses = new String[] {'user@acme.com'}; 
String[] ccAddresses = new String[] {'smith@gmail.com'};


// Assign the addresses for the To and CC lists to the mail object.
mail.setToAddresses(toAddresses);
mail.setCcAddresses(ccAddresses);

// Specify the address used when the recipients reply to the email. 
mail.setReplyTo('support@acme.com');

// Specify the name used as the display name.
mail.setSenderDisplayName('Salesforce Support');

// Specify the subject line for your email address.
mail.setSubject('New Case Created : ' + case.Id);

// Set to True if you want to BCC yourself on the email.
mail.setBccSender(false);

// Optionally append the salesforce.com email signature to the email.
// The email address of the user executing the Apex Code will be used.
mail.setUseSignature(false);

// Specify the text content of the email.
mail.setPlainTextBody('Your Case: ' + case.Id +' has been created.');

mail.setHtmlBody('Your case:<b> ' + case.Id +' </b>has been created.<p>'+
 'To view your case <a href=https://***yourInstance***.salesforce.com/'+case.Id+'>click here.</a>');

// Send the email you have created.
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
} 


}

}