It depends on what level of customization you want. Some questions can be answered by ACT! Technical Support. They can show the tools needed to customize, but you will need to do the actual customization yourself. If you want more hands-on, detailed customization, then you can contact an ACT! Certified Consultant (ACC) in your area.
Note: Effective 6/1/13, Sage no longers provides support for the Act! software. This is now provided by Swiftpage.