IF your not sure about the gimp there is tonnes online in way of tutorials
The GIMP
Select the parts of the image you want to be visible. (Or select what you want to be transparent, and do Select?Invert.)
Do Layer?Mask?Add Layer Mask
In the dialog, set Initialize Layer Mark to 'Selection'
You can make further changes by painting directly on the layer mask. The layermask is just a grayscale image; black means fully transparent and white fully opaque.
The new mask is selected in the layer list. Before you save, click on the original layer to select it. (Otherwise, saving will only export the grayscale mask.)
When saving as TGA, the export procedure will tell you it needs to "Apply layer mask". Click 'Export'.
Another way to do is it right click on the layer and select "Add alpha channel" and use the Erase tool or Edit?Clear to erase the parts you want transparent.