What sort of model are you trying to trim out? A trainer requires a more forward balance point than an aerobatic model in order to be more stable for a new pilot.

EDIT- I just went to Google in put in model airplane trimming and got back a lot of good looking hits on the first two pages of listings. Two of them were the links above. Some others were more for general and training style models and they all have something to offer. Do the same search and read through those pages.

Also check out the links in the sticky threads at the top of the listings on this Aerodynamics forum as well as the Scratch Building forum for links to sites with information and calculators to help you determine your CG location.