I'm sewing a mediumweight denium,but I get puckered at the begining of serging, so my question is do I need to change the top blade or lower blade or needle?

Make sure needle is not dull and try lowering tension. See following videos.
https://www.youtube.com/watch?v=nIu7-Hy4HKg
https://www.youtube.com/watch?v=v9jKANRy0LA

1. Have lower blade even with the needle plate
2. Have upper blade overlap (like shears) at least 1/32 of an inch or less.
3. The blades must meet but not overlap too much or fabric will just jam
4. Move 1 blade at a time.
5. Run machine Without it being threaded to check progress

Depends on why the seam is puckering. Read this: http://www.amefird.com/wp-content/uploads/2010/01/Minimizing-Seam-Puckering-2-5-10.pdf
Offhand, it sounds like you need to reset the top tension properly.
http://quilting.about.com/od/piecingtechniques/a/How-to-Adjust-Sewing-Machine-Tension.htm
The top two photos here are the upper and underside of a single ply of muslin sewn with a balanced tension. The blue thread is on top, red in the bobbin: http://www.picturetrail.com/sfx/album/view/22521551 -- that will give you something to shoot for.

YOUR PUCKERING IS DO TO EXCESS TENSION. LOWER YOUR TENSION ON TOP. IF IT STILL IS DOING IT YOU MAY HAVE TO LOWER THE TENSION OF YOUR BOBBIN CARRIER.

Check tension and threading up first, sounds like top tension or thread catching somewhere. Also have you changed the needle, i change mine for EACH new sewing project, needles go blunt and need replacing every 4-6 hours of sewing. Lastly, press seams as you go, this improves appearance and blends stitching into the fabric,

not necesarily, can you see a knick in either of the two blades? use a magnifying glass and see if you can see any damage first.

Then try changing the fixed bottom blade only first, you may have got a replacment top blade in your accessory bag new with the machine.

The blades themselves are very high grade steel just like scissor blade so should last for quite a long time, provided they don't come into contact with a pin.

You can buy the bottom blade for your machine here http://www.sewingpartsonline.com/lower-knife-babylock-60544.aspx along with the top blade and other spares.

Also, remember to clean the blades of lint regularly and give them a wipe with a cloth moistened with a little clear sewing machine oil from time to time.

you don't say what make and model serger this is.

However, as a rule of thumb, set the blade to widest setting, then test serge and tighten up the top and bottom looper threads to encase the cut edge neatly. So you would need to turn the dials on the two right hand knobs to a higher number, just move each dial one number at a time and test serge again.

The other possibility is that the thread is not between the tension discs properly, so check your thread paths, when you pull on the thread below the tension dial, it should feel firm to pull.

I always test serge scraps when sewing a new fabric and adjust the tensions to suit it before beginning a new project as a lightweight fabric will behave differently to a heavy fabric.

I have the same problem with my Imagine and am trying to find someone with some real knowledge. I think the problem is that the forks holding the thread are not opening up as wide as the stitch width and this causes puckering. if you serge just off the material your stitch will probably be perfect but your blades will not touch the fabric. My sewing repair shop thinks I used my machine too much and that I should send it to babylock which would cost 500 to 600 . How silly is that. I am trying to find out how to adjust the forks or someone who can do it for me. If you find something please post answer.

From "Serge With Confidence" by Nancy Zieman:

Set the machine up for a 3-thread rolled edge. Normal width; length 1 - 2.

Use Woolly Nylon in the loopers & regular thread in the needle.

Lock or disengage the cutting blade.

Guide fabric along the right edge of the presser foot.

Stretch and equal amount from fron to back while serging.

Lower the differential feed, if available to about 0.6.

Serge at a slow to moderate speed.

Practice with scraps first.

After serging, gently stretch & release the fabric edge after serging.

This machine has differential feed (top tension is different from bottom and fabric feed rate can be different from bottom). turn the dials for tension and feed rates to adjust. Adjust bottom tension to tighten bottom threads and possibly speed up bottom feed rate. Tough to say without more info.

Good luck and please rate me.

From White Sewing web site.

"Are wavy fabrics making you seasick? Do puckers leave a sour taste in your mouth? Then you need differential feed. It eliminates distortion and wavy seams when serging knits, plus it prevents puckers and pulled seams on lightweight fabrics. Turn the dial again for instant gathers.