Lets assume you own a Mac Mini late 2011 or 2012 with OS X 10.7 Lion or 10.8 Mountain Lion and you want to exchange the slow HDD which came with your Mac in favor of a SSD. You will need the following for the whole process:

Prerequisites:
– an Apple ID
– a downloaded, not yet installed version of OS X 10.8 Mountain Lion
– 8GB or larger USB flash drive
– a screwdriver set
– 45 additional minutes

Now simply follow the instructions:
– Do not (!) format your SSD with any partition before plugging it in.
– Exchange the drive: http://www.youtube.com/watch?v=gBongunj4pc
– Plug in the USB flash drive created as illustrated here: http://arstechnica.com/apple/2012/07/how-to-create-a-bootable-backup-mountain-lion-install-disk/

Have fun!