You can either hang it off the tank or you can put it in your sump, depends on the model you buy. Personally, i like the in sump ones because sumps are such a great place to hide equipment, you get a larger water volume and you can set a fuge down there.
And yes, all you need is a skimmer and live rock. You can also add a fuge, with is a place for macro algae to grow which will also help your water quality. The fuge also provides a safe place for pods to grow which are washed back out the main tank where they are free fish food. However, this isnt a must have addition, just something nice.
I would suggest the reef octopus skimmers, they are a great brand for the money. I would be wary of cheap skimmers, they are exactly that. Skimmers are very much so a you-get-what-you-pay-for commodity.