There are, of course, endless resources online for this sort of thing. Idrawgirls.com has a great range of video tutorials, particularly for an insight into the process of digital painting. I will no doubt go back to this site when I reach the stage of creating final, refined digital paintings of my characters.
At this stage, however, I'm mainly working on creating a range of thumbnail sketches and silhouettes, so for the moment I don't need to worry too much about detailed pieces.
The concept artist Mark Molnar has uploaded a serious of videos detailing his character design process which has been very useful to me. He begins by using a plain round brush in Photoshop and creating very loose thumbnails, with the intention of creating a range of shapes, both male and female. He then takes the ones he finds most interesting, and adds more definition using a 50% grey brush. He then erases parts around the silhouettes to add more suggestions of detail and shape. He then uses the same brush with the added 'transfer' attribute to push details back and create more of a sense of atmosphere.
When he has finished, he adds a background to make it more presentable. He advises that you should always try to make your work, no matter what stage it's at, look interesting and well-presented. This is definitely something I need to do more, as I tend to be messy and unorganised, particularly with early draft work.
He gives lots of useful tips and advice during this video on the process as a whole. I will be checking out the whole series to further develop my understanding.