Have you used an overlocker before? If not, I'd suggest that you obtain a user manual for it as they are completely different to a sewing machine and at some point you will need to rethread it. You can purchase one from here
This machine makes three and four thread seams, which is a combination of two needles and two loopers. For the three thread seam you can use either the left or right needle which gives you different sized seams. For the four thread you use both needles, one goes through the middle of the seam and the other is on the left side securing the seam.
On most overlockers you are able to move the cutting blade too and this will reduce or increase the amount of fabric being trimmed from the right side.
This page http://sewing.about.com/od/sergersoverlock/ss/serger.htm
will give you some general information on sergers/overlockers too.
So adjusting tension is a matter of changing the tension on the top and lower looper threads to close up against the cut edge, or moving the blade over to make the seam a little wider or narrower. You may also need to finess the left needle tension so that the seam doesn't pull apart, I always test serge two layers and then pull open from the right side and see if the needle thread is showing (it will show a little bit but you dont want it so loose that it pulls open. Only tighten the needles in very small increments though, as you'll break threads and needles if you turn them up too tight.
Hard to explain without seeing what your machine is seaming now. But as a rule of thumb, start with all four tension dials (or 3 if doing a three thread seam) on 5. If the tension dials are correctly calibrated then this should give you a pretty good seam. You may wish to lengthen the stitches, I usually work with stitch length of 2.5 (same as your sewing machine stitch length) Stitch length will be a dial on the right side beside the flywheel or in front of the blade area. The blade adjustment should be a little knob near the blade area too. There will be a lower fixed blade and a moving upper blade which sits against the lower one, they work like scissors to trim the fabric. So to adjust the blade, you will need to take the pressure off the upper blade, then wind the knob to move it left or right.
Different weights of fabric will behave differently on the overlocker so you do need to adjust tension for each new project. For example, if seaming a jersey knit you'd use a four thread seam, this gives elasticity and strength and you can join two garment pieces with this seam. You'd probably make a 6mm or 7mm wide seam, the left needle will secure the fabric and the loopers will encase the fabric smoothly while the right needle secures them and gives extra strenth to the seam.
But on organza for example, I would make a very narrow three thread seam using the right needle as the organza will roll inside the overlocking if you cut the fabric too wide.
And on something like curtains I would neaten the edge with a wide 3 thread seam using the left needle to get a very wide seam, the fabric wont roll so you'd need to loosen the two loopers a little to smoothly encase the cut edge.
For example, on this image above you can see that the looper threads are laying a little off the edge of the fabric in places, particularly the lower looper, (the side that looks like Y's where you can't really see the middle needle thread), so I would tighten the lower looper about .5 on its dial, then test again.
I hope that this makes sense to you, you will need to test serge, and adjust one dial at a time, test again and look at the result, then maybe adjust another thread until you are getting a smooth looking seam.