How to get rid of weeds?
There are many different types of weeds that could be in your lawn. Some have narrow leaves while others like dandelions have wider leaves. Knowing which weeds you have will help you know how the weeds spread (i.e. seed or underground roots) and the best time to treat them. Below is a link to the msu weed site which will give you more detail about the typical turf weeds in Michigan. http://www.msuturfweeds.net/
Assuming most of your weeds are broad leaf (i.e. dandelions), then you can purchase a broad leaf weed killer product. Make sure you follow the label for application rate and timing .
One more thing to consider is how to create a lawn that minimizes weeds. The healthier and thicker your lawn, the fewer weeds and the less chance the weed seeds can find soil in which to germinate. Below is a link to the MSU web site where you can find more info about being a smart gardener with your lawn. Hope that helps!