I am recently moved from eclipse to intellij to program in java. I have the following questions:
When you create the project you are asked for a project location:
You choose that location and it does not have to be in the "IdeaProjects/ folder".
IntelliJ provides a help topic titled Migrating From Eclipse to IntelliJ IDEA which maps Eclipse concepts to IntelliJ concepts. Might be useful to read that to help you apply what you expect (based on how Eclipse behaves/is desigend) to how IntelliJ behaves.
Re your other questions:
File > Project Structure > Librariesthen ...
Javaand a file browser will be opened and you can use this to browse to the JAR you want to include in your project.
File > New > Project from existing sources ...then