how to make a field value read only for everyone except admins after creating a ticket.

Hi Mamidipaka - Welcome to the Atlassian Community!

Your best best is a third party app from the Marketplace, like ScriptRunner Behaviours. 

Or if there are not a lot of statuses in your workflow, here is what I do. Create separate Edit screen and view screen for the project/space and even work type if applicable. 

Then on the status that you want some users to be able to update, create a self-looping transition on the status in the workflow (a transition that goes from a status to the same status). Then create a screen that just has the field you want to be able to update (of fields if more than one). Then add the screen to the transition by adding a rule to the Transition. Then add another rule to the transition to Restrict access to who can execute the transition. Then add the users who should be able to do the update. 

Then when the work item is in the status, the user can execute the transition which causes the popup screen with the field to change. Add the value and click the button to save/complete the transition. You can add this same process to multiple statuses in the workflow if needed. 

Hi @Mamidipaka Pavan Kumar,

Welcome to Atlassian Community!

Jira doesn't have field level security, so you would either have separate screens for creating the work item and then have a circular status that only admins can access that shows a screen with the specific fields, or use a Marketplace app that can do field security.