@kis , you write that you can program in Java. This means that you can program at all.
So just read K & R, and then man 3 intro, man stdlib.h, man stdio.h, man string.h, man ctype.h ... , well, untwist SEE ALSO in them.
Of course, over the past 30 years, some changes (compared to K & R) have appeared, but IMHO with a known base, they are easily perceived when reading /usr/include/*.h .
In the appendage is to read the "Practice of programming" Author: Brian Kernighan. Rob Pike, there are great examples in different languages, but mostly in Xi.
Is it worth learning C ++?
Almost certainly you will have to deal with other people's programs on it. Therefore, it will be necessary to study, but it is better to have already mastered C, since several hundred pages (as in K & R) will not work here.