Well first of all I need to look at previous pictures of myself to determine what kind of change this is, if my pictures remained male then chances are that nobody would know me as a female and I don't have long before my parents try to kick my new female body wearing their sons clothes out of their house. In this situation I would write a note saying how I had left to hang out for the day then climb out the window with a few things and hide in the shed as I try to figure out what exactly is going on, checking the news and such on my iPod to see if anything related to gender swapping has been mentioned.
I would also at this point probably use a mirror to determine just how hot I am and what I'll be able to pull off with this new body.
However in the case that my pictures have changed and I appear female in all of them then I'd check my friends online and see if anything has been changed, look out for any possible relationship that somehow got started and determine how I'd approach the coming school day, checking my homework and clothing, inspecting my drawers for anything out of the ordinary, seeing as I'm female now. I'd take my measuring tape and learn my sizes again so I don't end up in an embarrassing situation where I don't know what bra size I wear.
Then I would proceed to live life in what I'd hope to be a decently cute body. I'd also probably learn how to do my hair.