Shih Tzu are one of the more challenging breeds to show successfully. As well as all the general rules and regulations, breed standard, dog training and ringcraft that need to be learned, the presentation of a long coated breed is a challenge that has to be met day in day out and not just on the day of a show. A long coat that isn't cared for and groomed in the right way simply can't be magicked into good presentation the night before a show.
The best person to talk to in the first instance is the breeder of your dog (if you haven't yet chosen a puppy, find a breeder who knows what they are talking about with showing). You will probably find that they will be very happy that someone wants to show one of their dogs but also perhaps a little anxious for you to do a good job. They will be able to share grooming tips and such, some dogs coats are much easier to manage than others!
We have a fairly in depth article about grooming here, Canine Coat Care sell a DVD which gives lots of detail. Also you can pick up some tips and tricks by attending shows as a spectator. All 5 Shih Tzu breed clubs have shows that only Shih Tzu attend, our show dates and locations can be found here and the websites of other clubs can be found here. A comprehensive list of all shows around the country can be found here. These are a good way to find something local but note that Shih Tzu entries may be very low at shows other than breed club shows and general championship shows.