To create a sandbox account in PayPal, follow these steps:

Visit the website: Go to the PayPal Developer website at https://developer.paypal.com/  and click on Log In to Dashboard button from the top right corner to login in PayPal Developer section with your PayPal account credential.

If you do have not a PayPal account then first create a PayPal account.

Client ID and Client Secret: After successful log in PayPal developer section, go to Apps & Credentials and you find Default Client ID and Secret Key mentioned on the page that help you to integrate the PayPal Payment gateway into your application.

Create New Sandbox Account: After all the above processes are done successfully, you can find and create a new Sandbox account from the Sandbox account option and click on create account button. Here you can also choose your account type Individual (Buyer Account) or Business (Merchant Account). And select your country then click on Create button.

Edit Sandbox Account: After creating a new sandbox test account you can see it in the sandbox account list mentioned upper picture. You can also View/Edit the account on click on the three-dot line on the right side in the account row.

You can find your sandbox account Email ID and Password generate by the system. Here you can edit your credential.

You can use this Email ID and Password to make payments when integrating PayPal API into your application.

Also, Read How to integrate PayPal API in ASP.NET MVC Application?