It depends on the kind of weed. Annual weeds are most easily prevented by using a pre-emergent herbicide. Preen is a trade name of a per-emergent herbicide. This usually requires spring and late summer applications annually as the soil can contain seed banks that can last for many years. Also, new weed seeds will migrate into the area on the wind or with the assistance of birds and small mammals. Established perennial weeds are controlled best by digging but can also be controlled specially formulated herbicides or with the careful use of broad-spectrum herbicide such as glysophate. Roundup is one of the trade names of an herbicide that contains glysophate. Broad-spectrum herbicides kill most plants they come in contact with so must be used with extreme care around desirable plants. Mechanical weed barriers such as landscape fabric can also be effective depending on location. Good mulching practices around plants can also help to suppress weeds. When using any herbicide you should read and follow the label instructions for its use and for personal protection. Here is some information about herbicides that can been effective. http://www.ipm.ucdavis.edu/QT/weedcontrolcard.html
If you can provide us with more information about the types of weeds you have, we may be able to be more specific.