Just like GP said:
Work out.Keep it simple in the beginning-look up a starting program,or look up some exercises and tailor your own program according to your schedule. If you don't access to a gym,there are also plenty of ways to build muscle without equipment.
Diet is also important.Focus on eating protein-rich foods such meat, eggs, and dairy.
As for style, the single best thing you can do is to buy fitting clothes.Try sizing down first(if normally you're an M,try on a S). Just be careful not to look like a sausage or feel extremely uncomfortable.You could also look up mens' outfits on Google to get some inspiration.