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1.  Prepare to mow your lawn at least 20 times during the season,  every 5-7 days in the spring and every 7-10 days summer and fall.

2.  Fertilize, Weed and Insect control.  (Scott's 4 Step Program works great) basically its this
-April, Fertilize with crabgrass preemergent combo
-June, Fertilize with broadleaf weed control combo
-August, Fertilize with insect contort
-October, Winterize fertilizer and lime
3.  Overseed thin areas between August and October

4.  Water, Water and Water, it's just not going to grow without it! 1"-2"per week. 
Do this and your lawn will be a lot better.  Of course, every lawn is different and some of these recommendations may be altered to fit a particular situation, but a great place to start, without breaking the bank. 
Easy Lawn Care

If you don't want all the science about lawns then lets fix it really simple. Below we are assuming that you at least have some good soils.  If not a Lawn Renovation may be needed.  Also, this is for cool season grasses but can be adjusted accordingly to different environments.
Lawn MowingFertlizingLawn WateringOver Seeding
Easy Lawn Care
Lawn Renovation
Lawn Mowing