I have an older Mac Mini and I want to buy a new one.How do transfer everything from one to another?

Use Setup/migration assistant.

How to move data to your new Mac using Mountain Lion and earlier - Apple Support

or

Move your content to a new Mac - Apple Support

I would only migrate data, not apps. Reinstall apps on the new Mac. If you migrate during setup that is best. If you migrate after setup a new user is created with the migrated stuff

Re: Migration assistant created a new user when transferring files from a PC to mac

If yo are update from Snow Leopard or earlier some old apps may not work

Upgrading to 10.7 and above, don't forget Rosetta!

OS X- How to migrate data from another Mac using Mavericks

OS X- Manually migrating data from another Mac

Target Disk Mode

I suggest that you get things setup before you first turn on the new Mini so that you can use the Setup Assistant to perform the migration. But if that isn't possible then use Migration Assistant in the Utilities folder of the New Mini. Process is the same, but results are slightly different. MA cannot overwrite an existing user account so it will create a different user account unless your first user account is not using the same name as the account you will migrate.