hi,on most chainsaws,you will see two nuts on the chain bar side of the saw,these need to be loosened slightly,and then you will see a screw(flat head type usally)on the inner side of the chain bar,this will need a slight turn until you can just about give the chain a tug downwards from its bar,to check for tightness,about a few mm gap should be good.,then the two nuts will want tightening afterwards,this will also tighten the tension on the chain slightly a praction more,so you will need to check tension again untill the required amount is right.
some newer saws have the quick release knobs which just loosen,then a turn on the bigger adjustment knob,either direction on trial or error for tight or loosen.
garry
October 2010