When looking for a potting mix go for something with the Australian Standards Mark.
There are two basic types:
Premium - which has enough fertiliser added to it to last your plant for 2 to 3 months
Standard - which needs to have fertiliser added to it at the time of potting. This one is great if you have specific plants that need a particular type of fertiliser.
If you have an old mix which still has pretty good physical properties like drainage and water holding capacity, there are products you can use to rejuvenate the potting mix. One example is PotMate which has fertiliser and wetting agents added to it.
It is really worth spending a few extra dollars and buying yourself good potting mix for the best results.