There are several things that can cause grass to brown. The tips will brown if your mower blade isn't sharp enough. If you pick random blades and look closely, a dull mower blade tears the leaf instead of cutting it cleanly.
High nitrogen fertilizers can "burn" grass, causing it to brown. Fortunately, nitrogen leaves the soil fairly quickly. Here is a link to a fact sheet on fertilizing lawns that may help you.
Browning can also indicate insect activity. Grubs may feed on the roots of grass plants, causing the damaged areas to brown. Lift a bit of turf with a shovel and look at the roots. If the roots look damaged, dig around and see if there are grubs in the soil.
Penn State publishes a fact sheet on managing your lawn through the seasons that may help you. Finally, you can get a soil test kit at your local Penn State Extension to find out what nutrient deficiencies you may have and what type of fertilizer you may need.