I personally use photoshop. I created the older confessions on this blog. I believe Hayley used paint. She has made the post-hiatus announcement confessions.
I just copy the text given and put it over an image that represents it. i.e. If the confession is about Anne Boleyn then I might use a photo of Natalie Dormer or a portrait of Anne.
I usually color/edit the image then place the text over it.
You can do the simple text and imafe on a multitude of programs, including MS Paint for Windows or Paintbrush for Mac. Both are free to download.
I prefer Photoshop since it allows me the ability of coloring and editing. However it is a very expensive program and you can find cheaper, more limited photo editing software to use. Including some free online ones.
You can most certainly use a Mac. I use a MacBook Pro.