Hey bro, I am myself 18 and been lifting for about 2 years. You don't have to change you workout routine to define your muscles, but to lose weight - you have to. That's basically how your muscles get defined - low body fat percentage = defined muscle tone. Try making your workouts more intensive, higher reps - lower weights. Walking on a treadmill 20 - 30 minutes before lifting can help your metabolism and when it comes to food. Eat lots of proteins, when carbs make sure to get enough fiber in there. No sugar! Thats the basic "recipe" for weight loss without cardio. But of course, have in mind that everyone's different and what works for others may not work for you. Good luck bro!

Thanks a lot man, wasn't really expecting an answer out of this but I am very happy I got one. I will keep all of your tips in mind when I work out. Also, I usually eat 1-2 times a day because I mostly eat sandwiches at school and dinner when I get home. Should I split my meals up ?

There's some dispute between the experts on whether you should eat small meals throughout the day or not. There's so many ways to regulate your meal plan that there are no correct answer. But from my experience, I don't think it makes much difference whether you eat 8-10 meals a day or if you eat 4-6... As long as you get your wanted calorie intake and the right nutrition, I think you'll be fine either way. But try different methods, find out what works for you, experience is the greatest supplement!