Make a Plan.
No time to regret that you dint study properly in previous semesters.
Start with basics. C language. Understand all the small concepts. Yashwanth Kanetkar is the best one to start C.
After you are done, C++ is pretty much easy. You have to learn only the object oriented concepts.
Then comes the mighty Data Structures and Algorithm. I would suggest you to spend maximum amount of time on these. Linked lists and trees are very important. Concentrate on that.
Then an average understanding of Networks and OS would come in very handy in the interviews.
The overall process took almost 3 months for me.