Here are the key things you need to learn:
1. Memory Management - Java takes care of it all for you, whereas in C++ you have to manually allocate and de-allocate memory, otherwise you will have memory leaks.
2. Learn the Standard Template Library (STL). C++ has a much smaller library of pre-built code that Java. The STL is very good for some basic data structures like map, set, etc.
3. Pointers - Java has references, C++ has pointers. You need to learn how to use them safely.
4. Polymorphism - Similar to Java but syntax is a bit different
5. Operator Overloading - You can redefine operators such as "+" or "-" to make your code look cleaner. This can be dangerous, but very useful if done correctly.
I think that is a good start. Overall the transition to C++ should be relatively easy once you master memory management and pointers.