There is someone's project at google code which I want to clone using git. However, nothing of these worked:
git clone http://some_app.googlecode.com
git clone http://some_app.googlecode.com/svn
git clone http://some_app.googlecode.com/svn/trunk
git svn -s clone http://some_app.googlecode.com/svn/trunk
How do I do it?
Since, based on the URL, it looks like you are trying to access a subversion repository, you cannot use git in a simple way to get to it.
Theoretically you could use git-svn, which you looked at in your last example, however if you don't know about it already you probably want to learn more about git and svn first.
To check the code out of an svn repository you need to use svn, as in:
svn co http://some_app.googlecode.com/svn/trunk
If you really want to use git to access the subversion repository via git, then you need to init the repository first. The workflow looks something like:
git svn clone -s http://some_app.googlecode.com/svn/
The -s switch says to use the standard layout, and so will append trunk to it.