Serger is not stitching

... not quite sure what you mean by "not stitching", here are a few things to check. If the problem is more specific, please leave me a note with more details.
Let's start right from the beginning (for 4-thread overlock/serge):
1. According to the serger manual, ensure that the proper needles are being used. That they are inserted all the way up into their positions and that the flat side of the needle is facing away from you. The left needle will appear to be shorter that the right, that is OK, all is going perfectly.
2. Use four good quality spool of serger thread (good quality sewing thread is fine also) and place them on the spool or cone holders. There should be "little bucket looking things" that fit onto the cone pins to make the cones sit nice and straight, use them. Rattling thread will cause grief.
3. The first thread to be loaded onto the serger is the Upper Looper ...it is the big needle looking thing that swings left to right and back again above the sewing surface. Follow all the threading guided according to your manual, the threading guides are generally coded in a colour specific for this looper. Once the eye of the Upper Looper is threaded, place the thread under the presser foot and to the left. Presser foot down.
4. Next is the Lower Looper, the big needle looking thing that swings left to right and back again under the sewing surface. It can be a bit tricky in some sergers as the thread need to pass to the left and then back to the right under the sewing surface. Your manual should have a good diagram if you can not follow the threading guides on the serger itself. Again the threading guides are generally colour coded with a different colour but specific for the Lower Looper. Once the looper is threaded, it is important that the thread is placed directly OVER the Upper Looper as it comes out of the eye of the Lower Looper. Place the thread now under the presser foot and to the left. Presser foot down.
5. Thread the right needle front to back and place the thread under the presser foot and to the left. Presser foot down.
6 Thread the left needle front to back and place the thread under the presser foot and to the left. Presser foot down.
7. Hold the four thread ends firmly in the left hand and give each thread individually a good pull down over the serger and behind, this will ensure that the threads are all engaged in the tension dials.
8. Set the tensions to the middle number, although this may vary with the brand and model of your serger. If you have the manual check for the correct settings specific to your unit.
9. Set differential to 'normal', or '0' (if your unit has one)
10. Set stitch length to about 2 1/2 to 3.
11. Make sure that the blade is in cutting position. We can trouble shoot blade position later if needed.
12. Make sure that you have the overlocking plate installed ... not the rolled hem plate (if your serger has two plates). Some sergers use the same plate for both functions.
13. Using two layers of medium weight fabric do a test stitch, what is the result? Is a chain forming? Are there stitches on the fabric? Are they looking the way they should?
Hopefully this has helped you, please let me know.
Cheers.