To apply for the Salesforce Admin Certification exam, you have to visit certification.salesforce.com. After creating a login, you can choose the exam and a convenient exam location for you (You can also attend this exam online from your home in online proctored mode if you have a webcam). After that, pay the fees via the credit card to schedule the exam. You can schedule the exam according to the time slots available. Webassessor has test across all major cities on the globe. You can choose the nearest to you.
If you want to prepare for Salesforce Admin Certification exam, check out this Salesforce Certification program by Intellipaat.
You can watch this video on Salesforce by Intellipaat to know about Salesforce certification in a detailed manner: