Obviously the best place to look for help is the Koha documentation, most of which is stored on kohadocs.org. There is also a lot of good information on the Koha wiki, at koha.org/wiki.
If you don't find anything helpful in these locations, then you should turn to the Koha mailing lists: access http://www.koha.org/community/mailing-lists.html to see what is available. The best of these lists for most problem-solving is the general Koha list, which you can join by filling out the form on the mailing-lists page. If you turn to the lists for help, always remember to include some basic information:
which version of Koha
what operating system
information about relevant parameters settings -- for example, if you are having a cataloging problem, is marc "on" or "off"
any portion of your error logs (usually in /usr/local/koha/log) that seems relevant
Remember that Koha is an open community. You should always share your problems and solutions with everyone on the mailing list, no matter how trivial (or embarrassing) you may think the problem to be.
If you are completely lost, or want to make some changes to Koha that are beyond your ability, remember that there are some companies that offer Koha service. A list of such companies is kept at http://www.koha.org/support/pay.html.