Making an image partly transparent

[tm2383]

Hi there. Can someone who is an expert with Gimp help me with a problem? I have pasted an image of an iridology chart below. I want to remove the white from this image altogether, so that the white layer is totally transparent. When I overlay this on the image of an iris, I will therefore be able to see the black lines and labels, but not the white, which will be transparent, showing the image of the eye that I have overlaid it on.

Also, is there a way of changing the colour of the black lines to another colour? This isn't essential, but it would be good if I could do this.

If you are able to help with my question, could you give me easy directions about how to do this, as I am not a regular Gimp user.

Thanks!