@tanya004,
There were some posts about this before.... I think most people here and most (professional) graphic designers are using Adobe Photoshop. I personally use Macromedia Fireworks (which is pretty equivalent) and since Macromedia is now a subsidiary of Adobe it's all from the same company.
Then there is also Corel Draw/Photo Paint which has a similar functionality.
Since these software (I learned that the plural of software is software too, not softwares
) are relatively expensive (look at the companies' websites or on ebay) there are also some good open source graphic editors but I can't tell you much about these. You might wanna do a search here on the forums or on Google.