Fertilizing a lawn regularly will give it healthy green color as well as help prevent disease and weed problems. If a lawn is well fed and growing rapidly, disease spots on the grass blades are mowed off before they become visible. A thick lawn will prevent weeds from getting started.
Lawns need mainly nitrogen, phosphorus and potassium or NPK. The numbers on every fertilizer bag are the percentages of N P and K found in the bag. Different lawn grasses need different amounts of nitrogen with bent grass and Improved Bermudagrass needing the most and fescue and centipede grass needing the least. A general recommendation that will do well with most grasses is 0.75 pounds of actual nitrogen per 1000 square feet every 6 weeks during the growing season. Divide 0.75 by the decimal form of the percentage of nitrogen to find how many pounds of fertilizer to apply each time. For example, a 15-5-10 would require 0.75 / 0.15 which equals 5 pounds of fertilizer per 1000 square feet every six weeks. A different approach for cool season grasses is to fertilize once in early spring and again in late summer. In that case, divide 1.50 by the decimal form of the percentage of nitrogen. This approach requires a slow release, non-burning fertilizer such as one containing blood meal as the nitrogen source.
Regarding which fertilizer to use, a turf research project at Oregon State University comparing many different lawn fertilizers and ingredients determined that every one tested worked equally well after one month. The only difference was that blood meal was still working after three months.