Who better to get some good sound gardening tips than from the UK’s Royal Horticulture Society (most famous for the Chelsea Flower Show). The RHS has a great website that gives tips on common gardening activities from lawn mowing to pruning. While these can be specific to Britain, there is a good store of general information as well.

Useful sections include:
Top Pests
Top Diseases
The Plant Selector
Popular Plants
General Advice Section

For more specific information from societies closer to home check:
American Horticulture Society
National Garden Clubs
All America Rose Selections
The Garden Conservancy
National Gardening Association