Good preparation is essential for requisite confidence during placements.
For programming focus on c & c++/java.
Regularly visit the websites: geeksforgeeks.com codechef.com careercup.com
Revise basic operating system concepts like threads , deadlocks etc
Implement data structures: binary search, linear search,heap , merge, section sort, linked list, stack operations etc.
Revise dbms concepts: normalization, queries, etc
For GD and PI, there are a plethora of videos available on youtube
Prepare for aptitude by consulting the previous year question papers and patterns of the company you are preparing for.
Indiabix.com provides an online test simulator, that will evaluate your problem solving skills.
You can find very useful study material here.