I need to lose a lot of weight by september!?

Okay. I'm just really flustered!I need to lose about 100 pounds. I know this isn't realistic but I still want to lose weight! I just really don't know where to begin! I've been walking a mile and a half everyday with my dog...I have eaten fast food and I know that's bad. Could anyone give me some guidelines and tips to weight loss? Thanks in advance! All helpful answers are welcome. Haha.