You are 18 correct?
How tall are you? in feet ?
Do you have any type of work out equipment at home?
Now i am going to answer you question generally and maybe i can go more in depth if you answer the above questions
1. Eating Habits- The more you eat, the more you gain. Simple right. The less you eat, the less you gain. that is the false contrapositive statement. The last thing you want to do is put yourself in a position to become unhealthy. Chose Whole grain bread/cereal/rice/food/, low fat yogurt/cheese/food, and whole milk. You need to start small, you need to understand that this is not going to happen over night and you want to do this is the most correct manner possible. You are probably used to eating 3 meals a day(which most people are). The correct way is to have 6 small meals. Check some websites out for planning meals, there are lots. You need to fill yourself up at meal time with the good stuff so you have less of a craving for snacks which tend to be less healthy, quick, high in salt and fat.
2. Water. Drink it, a lot. Learn to Love it. drink it, live it, sleep it. Be it. It will be your friend. water will help you to lose weight. You body will use the water to break up the fat content and then it will make it easier for you to lose the weight. The recommended amount is 8 cups a day. I drink around 10. Refrain from drinking soda, coffee, slurpees, and alcohol.
3. Work out. Start out small. You have stairs in your house, use them. go up and down them as fast as you can well also being consistant. Push yourself as far as you can go, but not to far, because you body won't be used to it. You will know you limit. Write the number down and try and beat it in 2 days. Don't work the same muscles twice in a row. The next day, do some push ups and sit ups. I imagine that you can't do that many (as not even i can). Write that number down and beat it.
Don't forget to stretch a lot and have a warm up and a cool down. Your warm up could be a 5 min jog around the block or 100 jumping jacks (or whatever you think is acceptable). You cool down needs to be something to tell you body that you are done for the day. Don't just sit down right after your workout, it will cause you difficulty. Even if you feel like you can't go any more, you need to do a cool down. it could be just walking around with you hand on your head and kicking you legs. Once you feel you heartbeat going down, then you can start to rest. DRINK A LOT OF WATER WHILE WORKING OUT
It might be hard to fit a healthy or more active lifestyle into your current life, but don't forget about it. If you see the escalator, take the stairs. If you see a vending machine, buy the water. Do as much activities as you can