Photoshop is the most popular editing program out there, and there is also the free program Gimp that is similar but does not have all the same features. You can find tutorials on both to help you learn what you need to, to use them. Hope this helps
There is also a pretty cool online program called Aviary that you can also try out. Here is a link to the Aviary program Welcome to Aviary
Photoshop is the best for this, but it is pricey and has a learning curve. If you don't want to invest the time and money for PS, go with Photoshop Elements for <$100 and it will do most of the editing you will want to do. GIMP is a free alternative, but not near the support on the internet and still has a learning curve. Corel is cheaper than PS but not quite as robust.
I will jump on the Photoshop train with the rest of these people. I'm pretty much entirely self taught in PS but you can figure out anything about the program by searching google.
Honestly, at first you will try to do things in the least effective way possible. You will try to attempt to do things with tools that seem familiar but you are not entirely sure what they do and you will experiment but once you go searching for specific things you are looking to accomplish you will quickly realize how effective of a program PS really is.
One thing I would recommend when getting into PS is to really try to utilize the keyboard short cuts as they will cut your work time in half as you get more comfortable with the controls.
If you're looking to make graphics though I would strongly recommend Illustrator but if you purchase the Adobe Suite you will get programs that will easily integrate with one another. Make sure you have a computer that can handle the load size of these programs because you can demand a lot out of you computer if you're editing stuff like I am.
Photoshop like several people said is still the best if affordable. There are easy to follow videos on YouTube to learn.
However GIMP will do a similar job with the added advantage of it being free(Open Source Software) It has a large community of users and developers on forums that will attend to any questions you may have.
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