I was like that too, really.
But watching a lot of youtube videos of tutorials, tips and the like helped me.
And of course screwing around with photoshop yourself will make you understand most of it.
Like what certain buttons do to an image. I learned a lot from fellow members here too. There are other members who make signatures and asking them stuff helps too ^^
I make my signatures on my own. I render the images myself and make the backgrounds too. I rarely pick something of internet (only when I'm too lazy to do it myself but that's something that not happens often xD)
Yes, I do use Photoshop. Photoshop CS5 Extended Edition to be exact. I know there are newer version but I'm most comfortable with this one so I prefer this one.
As for the rounded corners, there's alittle trick to it and here's the youtube link I learned it from:
http://www.youtube.com/watch?v=dzO8vE8u7jk
It's really easy ^^