If you have weeds, first things first you want to prevent any new weeds from establishing themselves in your lawn. By applying a pre emergent twice a year, this sterilizes your weed seeds & thus prevents them from ever germinating. Note: this does not kill existing weeds, but only kills the weed seeds.The next step in eliminating weeds is to spot treat larger weeds, and apply weed & feed at the proper time of the year. With these three steps, you'll have a weed free lawn in no time!

If you have bare or brown spots, this usually indicates a sprinkler or watering issue.
Your sprinklers may need to be adjusted or repaired.
In other cases, this could be due to insects which feed on the lawns root system.

Typically thinning lawns are caused by improper sunlight from large trees or other sources of shade. Other times the cause is compacted soil that chokes the roots. This can be cured by regular aeration. Finally, thinning lawns could be due to tree roots competing will the grass roots for nutrients.

Yellow discoloration is usually casued by malnurishment, or by female dogs which have high-acid content in their urine. If it is yellow or discolored in patches this could be due to insects feeding on your lawns root system.