I recommend GIMP. It is available as a free download from GIMP.org and comparable to Photoshop in many ways.
However, it's important that you realize that although you can do plenty of creative things with image editing program, they should never be used as a substitute for good photography.
You will get much farther by learning photography itself well and using an image editing program as an occasional aid than depending on image editing to give you good images.
You can imitate good photography, but it is no where near the benefit of doing it all naturally and taking good photos.
However, it's important that you realize that although you can do plenty of creative things with image editing program, they should never be used as a substitute for good photography.
You will get much farther by learning photography itself well and using an image editing program as an occasional aid than depending on image editing to give you good images.
You can imitate good photography, but it is no where near the benefit of doing it all naturally and taking good photos.