The procedure to convert 15 digit salesforce Opportunity ID to 18 digit ID is as follows:
Go to setup
Now go to Customize|Opportunity
Now Select Fields
Now go to Opportunity Custom Fields & Relationship Section and select New Button as shown.
Now Select Field type as Formula.
Select Next Button
Step 2: Choose Output type
Enter Field label as Opportunity ID and Field Name is automatically filled when clicking
Select formula return Type as Text and press next Button
Paste the code as shown above and select Treat blank fields as blanks radio button in Blank Field Handling section.
Select Visible button as shown above and select next button
Now finally save the all settings.
Now go to Opportunity and open any record then we find 18 degit ID values as shown below.
The Formula Field appears on the Opportunity detail page and displays the 18 digital Character ID.
Courtesy – http://www.crmsalesforcetraining.com/converting-15-digit-salesforce-opportunity-id-18-digits/