So my first tip is:

The program you use matters a lot. The best program for graphic designing is without a doubt Photoshop. However you can use GIMP or Pixlr as well .

Now considering that I use Photoshop CC and haven't used GIMP in a while , I'll be covering Photoshop only.

The basic thing you need to keep in mind while using either of these programs is the tools used.

For example the tools you should know about are:

Photoshop: Quick selection tool

The Eraser tool

The gradient tool

Layer Mask function

Image adjustments: Hue &Saturation

Brightness and Contrast

Levels

Layer Styles

Filters

I'll be covering the first tool in the second chapter.