Android : Digital pen android development

I Didn't come across digital pens and android developments, i'm looking for to write my own app where if i have connected my phone to my digital pen i want my phone to receive the information and to be used in such as maybe for example,

add it to a form, generate an img, if numbers how many numbers written, from the app.

where or what is the best place to get started? (links also preferred) what areas i should be looking to and focusing on? is it possible?

thanks.