Do you know what type of grass you have?

If you want to kill weeds, use a liquid herbicide and spot spray the weeds individually. Weed-B-Gone is good for most lawns and for most broadleaf weeds (like dandelions). You can get it in a container that fits onto the end of you hose. You need a trigger to control the spray.

Lawn care is much (MUCH) less about fertilizer than it is about proper watering and mowing. This time of year you should be watering about once every other week unless you get an inch of rain. You will increase the frequency to about once per week in the hottest heat of summer. When you water, start by watering a full inch at one time. Measure that with cat food or tuna cans placed around the yard.

Most lawns look best when mowed at the mower's highest setting. Exceptions include bermuda, centipede, and bentgrass which should be mowed at the mower's lowest setting. Never bag the clippings. The clippings return nitrogen to the soil in the form of protein developed by the grass.

Chemical fertilizer may be applied in late spring and twice in the fall. Organic fertilizer may be applied any time, any day, or every day without fear of harming the grass.

By following these guidelines you may never need to use herbicide because your lawn will be too dense for weeds to penetrate.

And yes, it would be helpful to know where you live and what kind of grass you have.