I think Photoshop do a free ungrade from version 2 to 3 and a few extra plug ins. But this is still well down the list compared with the latest software which can be purchased for around £50. Although you can easily spend £1000. Sometimes older software, but newer than Photoshop 2 is given away with photographic magazines and computer hardware. I noticed that PS 3.2 and Serif 8 was available with the last Computer Shopper mag.
If I remember correctly, Curves isn't available with PS 2 unless you get the upgrade; but you can use Levels instead. This is probably easier to use for a beginner anyway.
There are quite a few free tutorials on the internet and I always recommend this one as being worth a look
Using the Photoshop Levels Tool
If you go into the full Tutorial range there is excellent advice ranging from complete novice to rather advanced techniques.