Well I'm by all means no expert, but it depends on what route you want to take. Assuming you don't have them already, there are two routes that I'm aware of:
1.) Buy rails which get installed into factory slots, then buy cross bars that are made to fit into the rails.
2.) Buy aftermarket cross bars that sit inside the door frames.
The first option is obviously more expensive, as you have to buy the rails and cross bars, but it means you have a cleaner setup, and allows you to easily remove the cross bars and leave the rails on. That means you don't have to deal with lining the cross bars up inside the door frames, or worry about scratching the paint.
The second option has less hardware, and less up front investment (in price and install time). You simply place the feet inside the door frames and tighten it down. Taken this route means you can just buy a simple Thule, Yakima, etc. universal cross bar set (with fitted feet) and whatever attachment you want. It also means you can remove them and go back to a completely clean roof.
I am likely taking the factory rail approach, as I had universal cross bars in my previous car, and I just want to take advantage of the fact that there is a built-in way to do it. My only hesitation is that I like the clean lines of the bare roof, but I'm personally not a huge fan of having the feet installed in the door frames.
Links would probably help:
The cheapest I've found the OEM rails are: https://www.parts.com/index.cfm?fuseacti...-VPLVR0086
If you have the pano roof, you need: https://www.parts.com/index.cfm?fuseacti...-VPLVR0090
The cross bars themselves: https://www.parts.com/index.cfm?fuseacti...-VPLVR0073
Or if you want to go with Thule for the bars (while using the OEM rails), you can select the "Roof Railing" option on this page: http://www.thule.com/en-us/us/buyers-gui...=iWM2dpveq
Otherwise select "Normal Roof", which means you'll need the fitted feet.