There are a number of non-chemcial options you can try first. Raise the mowing height to 3 inches. Cutting the grass short is stressful to it and with shorter leaf blades, there is less shading by the turf (meaning more sunlight hits the soil and more weed seeds germinate). If there are lots of weeds and your yard is on the small side, you could have everything killed off in August and then completely reseeded. I like turf type tall fescue. Many people prefer Kentucky bluegrass or perennial ryegrass.
I would recommend you bring some of the weeds into the Chester County Extension office to get help with identification. (http://extension.psu.edu/chester) Below is a link to herbicide recommendations. Ignore the product name and just look at the active ingredient. However, they are meaningless without knowing what weeds you have.