i want take admin certification exam where and how can i prepare ?

Hi @Shanmukha Srinivas Kuchimanchi, You can start with official with Trailhead:

Get to know more about certification with the below Link.

To Prepare Follow Below Links:

Trailhead: https://trailhead.salesforce.com/en/credentials/administrator

Trailmix: https://trailhead.salesforce.com/en/users/strailhead/trailmixes/prepare-for-your-salesforce-administrator-credential

Trail: https://trailhead.salesforce.com/content/learn/trails/administrator-certification-prep

Practise Test: https://developer.salesforce.com/files/ti/thi/THI-000393/administratorpracticetest.html

Register Exam in Webassessor: 

Step 1: Signup for a Webassessor Account, Make sure you Link your Salesforce Developer Org & Trailhead by entering the email Id assosiated with your Developer org during Sign up.

https://www.webassessor.com/salesforce.

Step 2: Register for an Exam (Online or Offine)

Step 3: Prepare & Give Certification Exam.

Later, You need to maintain it for every Year, by completing a Maintenance modules in Trailhead.

Please mark as a Best Solution, If your query is resolved.