Once your figure out what your doing with your images then select appropriate software.
Rushing our and spending as much on software as your camera is not a good idea.
Your Software choices are:
- What came with your camera
- Free software
- Commercial products
Until you have a better understanding of your needs stick with the free stuff.
The following programs are more than sufficient for 98% of PP that any photographer is likely to do.
a) The GIMP http://www.gimp.org/
(this has many plugins and has similar capabilities to Photoshop)
b) Picasa from google http://picasa.google.com/
c) Paint .net http://www.getpaint.net/
(Very lightweight, good for JPEGs and simple stuff)
A more comprehensive list is available here
And the missing 2% can be done using other programs (eg. Autostitch for panoramas
Digital Asset Management (DAM) programs
In addition to Aperture (for Mac) and Adobe Lightroom (PC and Mac) there are some free DAM software:
Picassa from Google can be used as a DAM
shows many other alternates for DAM software
There is an excellent book on the subject "The DAM book" if you really need to know