How do l fix our leaky kitchen faucet

Sounds like your water supply lines under sink where they connect to faucet unit are loose oo they didn't apply plumbers putty under the faucet unit it self

from where is it leaking

These faucets can leak in three different places:

1. Internally at the valve seat
   You'll see water dripping from the spout. This means the rubber washer is worn or damaged. To replace the washer, remove the decorative cap from the handle. Remove the retaining screw and pull off the handle. Remove the decorative cover, called an escutcheon, if there is one. Unscrew the packing nut counterclockwise. Unscrew the valve stem and remove it. Remove the screw that holds the rubber washer in place. Because these washers come in many shapes and sizes, you'll want to take the old one to the hardware store and buy an exact replacement. Be sure to buy two washers so you'll have one for the other valve. Replace the washer and reassemble the faucet.
   
2. Around the handle
   If you see water coming from below the handle, the packing nut is leaking. Older faucets have a string-like graphite packing material wrapped around the stem. Newer faucets use an O-ring. First try tightening the packing nut clockwise... about one-half turn. If this doesn't stop the leak, remove the packing nut and replace the packing or O-ring; then reassemble the faucet. If the valve is using the graphite packing material, it might be a clue that you should shop for a new faucet.
   
   
3. Between a pivoting spout and the body
   The spout might be held in place with a packing nut, similar to the faucet stem, or it might have a nut at the top of the spout. In either case, remove the spout and replace the packing material or the O-ring seal. Then reassemble the faucet.

Single lever faucets also are known as "washerless" faucets. They are easier to repair than compression faucets after you determine what type of faucet you have. There are three types of washerless faucets: ball, ceramic disk and cartridge.
Leaks are evident in two places... dripping from the spout and seeping from around the spout where it joins the body of the faucet.
Leaks from the spout indicate that the ball, disk or cartridge is leaking. To repair, turn off the water to the faucet, then remove the handle. The handles on ball-type faucets are held in place with a setscrew that requires an Allen wrench to loosen. Ceramic disk and cartridge-type handles usually are held in place by a screw hidden beneath a cap.
On ball and ceramic disk-type faucets, remove the ball or disk and replace the seals. You can buy replacement kits for these faucets at the hardware store or home center.
On cartridge-type faucets, remove the spout and pull out the retainer clip that holds the cartridge in place. To remove the cartridge, pull on the cartridge stem with a twisting motion. If it resists, install the handle so you can a better grip on the cartridge to pull it out. Take the cartridge to the hardware store or home center and purchase an exact replacement kit.
Reassemble the faucet according to directions.
Leaks at the spout-to-body joint indicate an O-ring failure. Disassemble the faucet as described above. Remove the spout sleeve and replace the O-rings. It's a good idea to replace these seals if you have the faucet disassembled for repair of the ball, disk or cartridge.

You have to lube the seals. New seals will be much firmer than leaking ones...but clean and lube is necessary.

Here's a link to show you how to get the spout off using "strap wrenches" The allow you to get the nut off at the base of the faucet spout without harming the chrome finish.

http://blog.trebacz.com/2012/12/how-to-fix-leaking-moen-kitchen-faucet.html


There is also a video at the bottom of the blog post showing how I did it.

if its leaking under the handle, all you need to do is remove the blue and red cap on the handle under you will find a 1/8 allen screw, loosen screw and remove handle and tighten the brass nut a little which will seat the packing underneath.

If you contact Moen, they will send you a new cartridge, as well as tool to replace it. ( 1-800-buy-moen )
No charge.

1. If it's "leaking" from the body then, the O-Rings between the body of the faucet got gunked up with minerals from your water. This interferes with the seal.
The answer is to take some grit cloth/very fine sandpaper and clean the inside of the spout body. Wipe clean. I would also suggest replacing the O-Rings on the faucet body, but at the very least wipe the O-rings and faucet body clean.
THEN: lubricate the inside of the spout body AND the O-Rings with silicone grease. It's translucent and readily available and the only thing I would ever use for a faucet repair. NOT plumber's grease, NOT WD-40.
Reassemble and enjoy.

2. If it's dripping, the you need a new cartridge. You can get them for FREE from the hardware store if you have the old cartridge to trade them. The problem is that Moen cartridges are water-sealed into place and you need a special tool to break that water seal. The special tool (just a tiny piece of specifically shaped plastic, really) comes, you guessed it - with the new cartridge.
Two possible solutions: a) bite the bullet and just buy the cartridge (~$25) or buy a Moen cartridge puller, usually available from the hardware store as well for about the same price. Ask, they might even lend you one.
The point of whichever tool you end up with is to rotate the cartridge itself inside the faucet body to break the water seal so that you can pull it out with a set of pliers. There will be a retaining clip to pull out as well.

www.plumbingpartsdepot.com/moen7300.gif
http://img2.timeinc.net/toh/i/a/plumbing/faucets-toilets-07.jpg
Good Luck.

you must remove the faucet form the sink. under the plate in the middle is 2 screws. remove the screws to remove the body.

Try removing cap of knob and tighten screw it could be loose. If that is not it it may need a new seal ring.