Search for the solution by plant type or by garden problem. You will find easily identifiable images of the most common problems. This then leads you to the most appropriate solution/s.
Not all plant problems are caused by insects or diseases. Sometimes an unhealthy plant is suffering from a nutrient deficiency or even too much of any one nutrient. Plant nutrient deficiencies often manifest as foliage discoloration or distortion. The following chart outlines some possible problems. Unfortunately many problems have similar symptoms and sometimes it is a combination of problems.
Be sure you eliminate the obvious before you kill your plants with kindness.