You must have skipped all of your science courses if you honestly think smoking cigarettes will keep you skinny. If you knew basic information about the human body, you would know that smoking does not and will not keep you skinny, nor healthy. Smoking suppresses appetite and will make you lose or maintain your current weight at first. But once your body realizes that you're not giving it enough nutrients to keep functioning properly, it will retain any fat you consume because your body thinks it is starving.
Why would you want to suppress your appetite? Eating is great. I love eating. I love food. I could never imagine myself purposely attempting to make myself not eat. You shouldn't try to cover up your basic human needs to consume food. You won't be happy, you won't be healthy and you definitely won't be skinny for long. Cigarettes don't have magical fat-burning powers, they just temporarily trick your body into thinking you already ate.
Why would you want to trade your health for being temporarily thin? By smoking cigarettes, you make yourself 25 times more likely to develop cancer. Is that what you want? Lung cancer? It's literally the most preventable type of cancer, because you have to (typically) voluntarily smoke cigarettes. You have a choice between cancer and health; why would you choose cancer?
Why would you want to look, feel and be unhealthy? I'm sure you've seen some of the physical effects of smoking. Your lungs become lined with tar and your once healthy pink lungs start turning black. Your teeth start yellowing, your skin wrinkles and becomes discolored, your eyesight weakens and so much more.
Why would you risk becoming addicted to nicotine? You can't just smoke during the summer to "stay skinny" and quit when it's not bikini season. It's not impossible to do that, but it's highly unlikely that you'll be able to quit once you're hooked and dependent on smoking. At that point, not only are you making yourself more likely to develop cancer, but you're also addicted to nicotine!
Why would you waste money on addictive, unhealthy, cancer-causing sticks? Do you know how much cigarettes cost? They're not exactly cheap. A pack a day would cost roughly $2450 a year (in Texas). That's not even factoring in the additional packs you would smoke since you'd be so addicted to them and would be unable to handle going an hour without one. You could literally save thousands of dollars for not smoking.
So let's recap. Smoking doesn't make/keep you skinny, smoking takes away your body's natural reaction to being hungry and makes you unable to take care of yourself nutritional-wise. Smoking increases your risks of developing cancer by 25 times, smoking makes you all-around unhealthy, smoking is addictive and smoking burns a hole in your wallet (pun intended). There are more points about the negative side effects of smoking, but I'm sure we're all familiar with them by now. So, I'll ask again. Is that what you want?