Getting into app development

So I've been wanting to start getting into app development, probably with iOS first but would like to try WP and Android as well. I'm only choosing iOS because I have an iPhone to work with as well as a MBPr 2013 (that must look awfully fanboyish but I just wanted to try out their ecosystem. Probs going with Windows when I need an upgrade as I already have a surface that I enjoy playing with).

Anywho back on topic. I do have experience with coding and I was wondering what people use to design their apps?

Do you just go straight to interface builder in XCode, or Photoshop or is there another program I've missed?

I'm most interested in the designing part as it's kind of a new area for me so yeah, what do you use to design your apps?

Also, are there any tips you have for starting off with developing apps?