Here are some resources that will help you become more comfortable with the most common general aviation GPS systems. Use these guides and courses with the RealSimGear and X-Plane for the best possible learning experience outside of the airplane.
AOPA VFR Course
This interactive course offered by AOPA introduces basic VFR usage of the Garmin 430/530 which is an important foundation for getting comfortable with these systems.
This King Schools course provides in depth coverage of all functions of the Garmin GNS430/530 for both VFR and IFR operations. The King courses use a combination of video and presentation style modules to teach the material and is available in an online format. This course costs $179 but keep an eye out for discounts and sales.
Gleim Aviation Garmin 530 Training Course
This $29 course covers all areas of the Garmin 530 and represents pretty good value. You can take the first unit for free to see what it's like.
Pilotworkshops GPS Manuals
These book style manuals provide step by step walk through of common operations for a variety of Garmin GPS products, including the 430/530.