photoshop works best. if your willing to take some time to find free tutorials online to teach basics and practice, it wont be too hard to pick up. theres also gimp. gimp is a free open source program like photoshop. it almost does everything photoshop will but in a more complicated manner. its not as user friendly if your starting out with a graphics editor.
for beginners with no experince with any of these programs i really recommend finding a torrent for microsofts "picture it" (i find the premium 9 version, their last of the series, the best). its made as a simple photo enhancer. you can use the program though to make your graphic designs and its VERY easy to do. the program is straight forward and very user friendly. anyone can pick it up and with some time make high grade/semi professional looking images. you can also use it as a segway into photoshop or gimp.
Read the directions, and directly you'll be directed in the right direction. 