It depends where you are, for example if you are in te US you have the choice of Lutron which you don’t elsewhere…
I would look at compatible technologies (Zwave, Insteon, RF via rfxcom, X10, etc) and look at the prices functionality and look of available devices.
I would search the forums per technology and see which are more popular/discussed
You can mix technologies!
I have legacy X10, RF stuff and Zwave. I would recommend Zwave since the network is meshed and you can extend further than with other technologies. Rfxcom is a good investment since you may be able to control or read a bunch of stuff you already own (thermometers, blinds…) and you may have less expensive options too. I would stay away from X10 if you have none.
Yes, for Zwave you will need a dongle, several are available, for X10 you may be able to use rfxcom (will interact only via rf, no power line) depending on wether you have a device that will bridge from rf to pl, otherwise there are a couple of bridges you can control with mochad. Keep in mind that only plugged (to the mains) Zwave devices will be able to extend the mesh.