In Java, unlike in Python, there are no straight-up functions; everything is a method in a class. The closest you can get are static methods, which are associated with the class as a whole and not any particular instance of it.
The closest you can get in Java to the Python code you posted is this:
public class Conversions {
public static double lbs2kg(double w) {
return 0.453592 * w;
}
}
I would definitely recommend learning the language from a book or tutorial such as those that others have suggested.
solved Trouble creating method to go from pounds to kilograms [closed]