I've been in your shoes before. I partied my butt off my freshman year and I have grades to prove it.
I ended up with a 2.0 the second semester of my freshman year and I had some very, very angry parents. It was weird because I had always been a straight-A student. My parents wanted to ship me back home and go to the University by my house. I said "No way!" I told myself that I would never get less than a B in any of my classes ever again. And I haven't. You need to set a goal of what GPA you want and stick to it. Even if that means you stay home on a Thursday night. Believe me....I know it sucks when your sisters are out partying and you need to catch up on your studying but you will be so happy with the results. Here's some things I did that help:
1) Read the books. It sounds so simple but I never bothered to read; I just reviewed lecture notes. Many professors pull test questions out of the book and the books will reinforce what the professor said during lecture.
2) Study at the library. Your apartment is too distracting. I have a computer, TV, and bed in my room which sound so much more appealing then a book. Go to the library and you'll have nothing else to do but study.
3) Take classes with friends that do well in school and have a study group with them. I had a study group for my Finance and Business Cornerstone class and I honestly believe that it is the only reason I did so well. Not only can they explain to you what you don't understand but you can help them as well. Teaching someone else is the best way to learn.
4) Let the professors know who you are. I'm not saying suck up to them! Go to their office hours if you don't understand something or ask if you could review with them the mistakes you made on the last test. Let them know that you are really intent on getting an A in their class. You'd be surprised how much they'll help you if they know you want to do well. I had an 89.6 in one my classes this semester and the teacher stresses that she will never round up. Well, she did for me.
5) Go to your classes.
6) If studying means you need to stay home instead of going to a party then that's the way it goes! The reason why you're at college is to get an education. Don't lose sight of that.
7) Do have fun. Maybe allow yourself one or two nights a week to have fun with friends. If you're always doing school work, you'll get burnt out and all your studying will be counter-productive.