Hello,
Read basic concepts in different
java related books. Visit tutorialspoint.com for basic concepts. On tutorialspoint.com concepts cleared in very easy language. You can understand very well in few time periods.
Also, visit spoj.com or codechef.com, there are lots of warmup problems and classical problems are available.
Try to select different problems on such kind of websites. And try to solve such problems in java.
More Practices
I hope this will help you