now， I have already learn someting about java, and I want use it. What can i do

Answer 1

You already did the right thing a minute ago by starting SoloLearn's Java course.
If you have any questions, just ask, but pls tell us exactly what you don't understand, maybe add some code to your description.
Have fun with the course ^^

Answer 2

learn the course.
write code on code playground (don't skip it)
modify your code to see what happen (don't be afraid if there is error. post a question,
community will help you)
try different combination.

Answer 3

Start coding on playground or PC
- Install JDK on PC.
- Add JDK to path
- Write code
- Compile code
javac source.java

Answer 4

For now learn the full course and after that learn Android development (best thing can be done by java)