How can i lose 45lbs ?

I am 21 years old and in the course of one year found myself 45lbs heavier. I used to have a gym regimen which used to maintain my 180lb figure. Now i weigh 225 and its really just embarring. in the past i was worried about wanting to gain weight to gain m. now i find that its harder to lose weight than it is to gain. what diet changes can i make. please be specific i know soda and junk food is a no no. as far as work out regimen i know i should enforce cardio but what type of regimen should i be on, i never really used to do much cardio.