By default, JIRA issue keys are of the format <project key>-<issue id>, eg. ABC-123. This general format is not changeable. However it is possible to alter the project key format in the jira-application.properties file, where it is defined as a regular expression:
As you can see, the default is two or more A-Z characters. If, for instance, you wanted to allow numbers (eg. 'ABC2-123'), you would change the regexp to ([A-Z][A-Z0-9]+).
Also, although JIRA normally starts counting from 1 ('ABC-1', 'ABC-2' etc) you can adjust the starting count by editing the project.pcounter row in the database. JIRA caches this value in memory, so first shutdown your JIRA instance before updating the database. Then make the change in the database, then restart JIRA.
You should be able to find this file in your JIRA installation directory under:
You may need to search hidden files and folders for it to appear.