Basic concept - OOPs
How to create - Class & Object
Constructor - Important
Conditional & Looping statements
Exception Handling
Java File operations - File input/output-read/write [File I/O]
Assertions
Inheritance
Up-casting & Down-casting
Access Modifier
How to call methods (Static & non-Static)
String Handling
Above mentioned topics are most important to kick start with Java+Selenium.