How to automatically add products to opportunity in Salesforce?

1. Go to:
Salesforce Classic: Click Setup | Create | Workflow & Approvals | Process Builder | New
Lightning Experience: Click on the Gear Cog icon | Process Automation | Process Builder | New

2. The process starts when: A record changes | Save.
3. Click Add Object.
4. Select Object: Opportunity.
5. Start the Process: When a record is created.
6. Criteria: No criteria—just execute the actions!
Note: If you have multiple record types for opportunities, and you wish to add different products for different opportunities record types, then use the criteria:
[Opportunity].RecordTypeID Equals ID <18-digit record type ID>
The 18 digit record type ID can be found by navigating to:
1. Gear icon | Setup | Object Manager | (object) | Record Type
2. Click on the record type that you want to be added on the criteria

Sample URL: https://ap15.lightning.force.com/lightning/setup/ObjectManager/Account/RecordTypes/0122v000001v0u5AAA/view
(What you're looking for is the bold text)
Note: The 18 digit record type ID can be found on the address link above and is only available in Lightning Experience.
7. Click Add Action.
8. Click Action Type: Update Records | Record: Select the Opportunity record that started your process.
Criteria for Updating Records: No criteria—just update the records.
9. Select an Existing price book: Field: Price Book ID Type: ID Value: <Price Book ID>
10. Save and add a New Action.
11. Click Action Type: Create a Records | Record Type: Opportunity Product.
Set the following Field Values:
a) Field: Opportunity ID Type: Field Reference Value: [Opportunity].Id
b) Field: Product ID Type: ID Value: <Product ID>
c) Field: Total Price Type: Currency Value: <amount>
12. Repeat steps 10 and 11 to add more Products.
a) Process will fail if the user tries to add a product that has already been archived
13. Activate Process