After joined the community, you go to post page then you will see the page like the following image. [Highlight with red rectangle] Post to:, You choose between posting the entry on your livejournal homepage or in community.
My case, I have joined 2 communities - jaeho_detox and yoosu_yongwonhi. So in the option list, there are three choices for me.
I hope this can help you Sara.