Weeding
The best way to weed is buy hand. Do not attempt to add any weed killer to your grow area unless you know more about the weed killer you are using and how it reacts with cannabis. Some weed killers say that they will protect your plant and only kill the weeds around but this is not very agreeable with marijuana plants. If you want to test weed killers then it is suggested that you may create a small patch with one clone to see how the weed killer reacts with it. For now, do your weeding by hand.
When you have weeded a grow area your plant will grow much better without having to worry about competition for light.
Generally throughout the plants grow cycle you will have to weed the patch every week for the first 2 - 3 weeks and then once every month until as such time as the competition has been left truly and well behind. Some grow areas may be weed free in March, but come June the area may not seem like it has been weeded at all because of the speed of weed growth. Whenever you visit your plants make sure you pull up a few weeds as it does help.
If you feel that your area is very dense with weeds then you may consider a ground cover. A ground cover is basically a cover that is placed on the ground with holes in it that the marijuana plant grows up through. Covers can be anything from plastic bin liners to sheets of paper. Of course this does not look good if you want to create a stealth grow area, but it does keep the weeds down.