Enabling Read-only permission for a field

If you have added a field to an object form and you want that no user can make any change to the field, you can make the field read-only for the users of the required profiles.

To enable read-only permission for a field added to a form for the required profiles

1. Navigate to Setup > Manage Users > Profiles.
    A list of all profiles in your Salesforce.com organization is displayed.
2. In the Profile Name column, click the required profile.
3. In the Field-Level Security section, select the object that you want to modify and click View.
4. Click Edit.
5. Select the Read-Only check box for the required field.
6. Click Save.
7. Repeat step 2 through step 6 for each profile.