is a good place to start. Versions of MySQL are available for all popular operating systems, and even some unpopular ones.
Any SQL book will teach you WAY more than you want to know. Start simple. Create a database. Create a single table with an ID column (primary key, auto-increment), a temperature column and a datetime column.
Create some kind of script (PHP comes to mind) that can update the table in the database. The Arduino can then GET that script with a temperature, and the script can figure out when it happened.
I developed something very similar over the weekend. I can share the database and table construction file and the script if you want.