No sewing machine does a true overlock stitch, that is what sergers are for, they trim and form a stitch on the edge of the fabric using 1 or 2 needles and two loopers. Costs more for an additional machine but the finish is much more like a shop bought item.
Some sewing machines have an overcasting or interlock stitch which looks like stitch 10 in this picture above. Your machine may have this stitch?
If not, does it have a three step zig zag? It looks like a zig zag but the needle goes into the fabric three times as it forms each leg of the zig zag. This stitch is great for neatening a raw edge and I use this a lot myself. Especially on wovens as it is always faster to seam with this rather than the overcasting stitch where the feed dogs move the fabric forwards, then backwards, then forward to make the stitch.
The sewing machine made overcasting stitch can be used to sew knit fabrics, attach rib around an neck line and neaten raw edges but it is always going to take longer than a serger and there is no trimming function so its never going to look like a shop made overlocked item. However, you can sew knit fabrics with a sewing macine with a bit of patience as long as the machine has some "Forwards/backwards" type stitches as these put elasticity into the seams - if you seam knit with a straight stitch, seams can break when under tension during wear.
And use a twin stretch needle and straight sttich to sew the hems, this looks a bit like a cover stitch and has a bit of give in it too.
Hope this helps you a little. You can find some good sewing general info on www.sewing.about.com
or invest in a good sewing book. I refer to my Vogue Sewing Manual and my Singer Sewing Secrets often. Or take a sewing class, its a great way to learn new things, get enthused and have fun.