Under the Help heading on the menu bar, you'll find plenty of information. By default, tooltips are activated so that when you pause your mouse cursor over an item, a small tooltip will be shown. Click Help, and you'll see Tips checked on. Just below the word Tips, there is something else you might find useful. It's called Extended Tips. Turning that on will give you slightly more detailed tooltips.
If you are looking for help on a specific topic, choose Contents to open up the OpenOffice help screen. The various tabs at the top left of the help screen let you search for topics by application with the Contents tab, alphabetically using the Index tab, and by keyword using the Find tab. You can even set bookmarks under the Bookmarks tab for those topics you regularly access.