When you post to this community, you'll see that you have the option of editing and tagging your entry. Clicking on 'edit this entry's tags' immediately after posting, or on the tag icon by your post, takes you to the tag menu. Use the scroll bar to choose the appropriate tag(s), remembering that the more tags you use, the more easily others can find your entry; multiple tags can be chosen by holding down the control key as you click the options. Then click on 'Save', et voila! your entry is tagged.
If you haven't tagged your entry, and later find that it's been tagged in a way you think is inappropriate, feel free to make the appropriate changes. Remember to search carefully through the tags - sometimes it can take a couple of passes through to find the tag(s) most appropriate for your entry; if you really can't find a tag that describes your post, simply enter an appropriate description in the text box (next to the 'Save' button).
[EDIT]: I am reminded that mods/maintainers are the people to prod about incorrect tags. Apologies for misleading you.[/EDIT]
You can find out more about tagging by clicking on the tags attached to this entry, or by looking at LJ's how-to pages.