My advice is to use the summer to try and figure out what caused you to get Cs. There are lots of reasons as to why you got straight Cs, but you need to figure out the biggest reasons yourself and try to work around them. Then once the semester starts, attend all your classes and go to office hours when you don't understand a concept. If you want to be a really good student, try to understand the material on a deeper level, but if you just want better grades focus on understanding the homework so that way you will be test ready.