I'm going to go out on a limb here and assume that you not only want to lose thirty pounds, but that you would like to keep it off. Instead of thinking it as "reaching my goal by summer", think of it as making it a liftime commitment. Yes, I know that sounds daunting, but allow me to explain why:
So often I see women who want to reach a goal of losing weight for their high school reunion, wedding, summer, pick something, and then fall back into their bad habits after the event is over. One year later after they have gained all the weight back and then some, they are kicking themselves for backsliding. They had "reached their goal" and had not thought about what comes next, so they just eased back into their old comfortable habits.
However, it sounds like you are ready to make a change, but remember this is a journey, not a destination. So, with that said allow me to make some recommendations:
Yes, cardio may be helpful for losing weight, but what you really want to do is change your body composition. This is where weight training comes in. By building your muscles your body becomes more metabolically active (burns more calories). Think of it as turbo- charging your system.
Weight training will also teach you that you are stronger than you think you are. It will give you a measure of confidence and inner strength you never had before.
Weight training will help to give you stronger bones which will help you as you get older (yeah I know you're only twenty now, but think long term). I'm fifty and osteoporosis is nowhere in sight.
Skinny girls are a dime a dozen. Anyone can starve themselves and spend hours on the treadmill. Strong, fit women are impressive and send a message to those around them. Be one of the few, not one of the many.
What to do:
Yeah, it's daunting with so many articles on this and other sites. But deep in your heart you know what you shouldn't be eating. Cut out the fried foods. Kick the fast food habit and start making yourself healthy meals. Eat 5 to 6 small meals per day. Cut out the simple garbage carbs: chips, fries, pastries, sweets and soft drinks. But you really already knew this, the hard part is execution.
If you are weak-willed enlist the support of someone close. Tell them what your plans are. Ask them to help keep you on track. During this time, educate yourself on nutrition and training. There is plenty of information on this and other websites. And don't be afraid to ask questions. Many of the women here have been down the same road you are starting on and can tell you what worked for them.
Which brings me to another point, and a very important one. We are all different. What works for me may not work for you, so you will have to experiment. If something doesn't work, don't get discouraged, just try something else. But don't give up. Never give up, just adapt.
Consistency is also very important. Whatever you choose to do, whether it's cardio, weights, yoga, whatever it is, do it consistently. That means you have to MAKE TIME, not find time. Don't try to work out when you have time. Make the time to work out.
I've been doing this for twenty-something years, so I've got a little insight into all of this. Believe me when I say that this can be one of the best descisions of your life.
Remember...a journey, not a destination.