Take a full XML export from the "backup and import" admin.
There are other methods of exporting data from Jira, but that's the only one that will really get everything, the others are limited.
You might get a more useful response if you explained why you want to do this though?
If you want the data in an an immediately useful format you could try the Intelligent Reports plugin. You design the report in Microsoft Word, and select the data you want to fill in, in this case issues and comments, with the simple point and click interface. There is no scripting or API to learn.
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