If just for a short while, I'd probably just go out and socialize, to see how different it is being a guy instead of a girl. Dress up in some men's clothes, go hang out, that kind of thing. To be honest I don't know if I'd check out my new body much; I'm a prude and it'd probably embarrass me. ^^;
If for the long term? Just be myself, but as a guy instead. I imagine it will change very little about me besides my wardrobe and how the world reacts to me. Then again, what do I know? D: