Lets remember guys that this is a thread about things needed to start sig making, not how we got Photoshop.
As the others have said before, Gimp is a great place to start. Though not as powerful as Adobe Photoshop, it certainly has enough features and versatility to keep you busy for quite some time. That and it's FREE!!!!!!
What I would do is download Gimp, and toy around with it a little, then check some tutorials. I found several good ones on Youtube that helped me a lot. Text sigs can be a lot of fun, so what you could do is search on Youtube for "Gimp text effects" or something similar: that should get you off to a good start.
Hope it works out for ya!