First I would look at the space between the brake pad and the rim.
It should be close. Maybe 1/8". Not more than 1/4".
The cable running along the frame top tube, should not be slack.
The brake hand lever needs to activate the cable within 1/4" of movement.
If any of these are not as indicated, you need to tighten the cable.
Grab a set of allen wrenches and loosen the bolt that holds the brake cable tight on the brake arm. Loosen it enough (don't remove it) to be able to pull the loose end of cable tighter. Again, look at the brake pads. Try to get them 1/4" away from the rim or closer.
Tighten the bolt to hold the cable in place.
Too close to the rim?
Adjust the micro adjuster on the brake hand lever to fine tune.
Note: make sure your rims are clean and the brake pads are clean and flat against the rim (or slightly angled so the front of the pad touches the rim before the rear of the pad does )
When in doubt, or you get crazy frustrated, talk to your local [http://mikefixmybike.weebly.com/|mechanic].