If you lift heavy and train hardcore, then fish oil is the best additional supplement (it is reduces joint pain dramatically, which you will have if you lift heavy)
But, if you training is not that intense, then you will see better results from a simple creatine monohydrate. Creatine is dirt cheap.
Buy both, get a cheap creatine (universal creatine is good, they use german imported creatine, not low quality chinese shit); and get a good fish oil.
And of course, eat shit loads of good food