Thanks for writing. In most cases, you'll either have to manually remove the weeds (pulling by hand, using a hand weeder or hoe, etc.) or use an herbicide to spray the weeds individually (alternatively, you can dip a paintbrush in herbicide and apply it to each weed that way).
Once you've gotten rid of the weeds, help prevent further outbreaks by using a pre-emergence herbicide (there are synthetic and organic types available) in spring and spread 2-3 inches of mulch of your planting areas. That can keep a large number weeds at bay.
---Justin, Senior Garden Editor, BHG.com