The trick is balance your use of references, try not to use other people's art too much, you risk hampering the development of your own unique style though you will improve more slowely than if you just spam references. I prefer to use real-life references whenever I need assistance sketching. Also I can't stress enough how ridiculously awesome using a graphics tablet is (Wacom Intuos ike Tripsis said is the way to go) and if you can't get your hands on photoshop, just download SAI, it's free and has a nice feel, don't use it without a tablet though, it's near impossible.