Paul, I have a 2015 Sienna LE and a Scamp 16 on order.

The Sienna needs a number of things to prep it to tow and if none of this has been done, you will have to scramble to get it all finished in a week. In fact just getting the stuff ordered online will take about a week.

1.  Hitch receiver.  This is actually the easiest thing on this list and it sounds like you have it already.  If not, it bolts on and one can even DIY (even doing it alone although it helps to have two people). It must be torqued properly and torque checked sometime after install.
    
2. Ball mount (AKA draw bar).  The top of the 2 inch ball needs to be about 21 inches above the ground to get the trailer level when hooked to the van.  On the Sienna, this requires a draw bar with a rise of about 7-8 inches (which is pretty much the most rise you can get).  The rear of the Sienna will sink and inch or so when the trailer if hooked up so take that into account.  Sometimes it seems that trial and error are the only way to get a trailer level while towing but an adjustable draw bar is a good option too.  BTW, I am pretty sure that with this much rise, the tailgate will not open when the trailer is hooked up because it hits the trailer’s tongue jack.  A longer drawbar might allow the tailgate to open but is not recommended. The jack can be relocated or a removable type can be used instead.
    
3. Trailer lights.  The Sienna does NOT have pre-wiring for trailer lights so you need a converter / harness that does not put additional electrical load on the van’s lighting circuit.  These require removing the tail light assemblies (be careful not to crack them during removal) and running a fused power connection to the battery. The harness has a 4 wire trailer connector that the instructions tell you to run over the tailgate weather stripping, but you will be using a 7 pin (see item 5 below). These are for the 2015 non-SE models, be sure and match the harness to your exact vehicle.
   http://www.etrailer.com/Custom-Fit-Vehicle-Wiring/Toyota/Sienna/2015/C56...
   http://www.etrailer.com/Custom-Fit-Vehicle-Wiring/Toyota/Sienna/2015/118...
    
4. Trailer Brake Controller.  This is the most difficult item you need to add, and again, the Sienna is not prewired for one.  IMHO, either the Tekonsha Prodigy P2 or P3 are the no-brainer choices (P3 was my choice).  You need to run another power connection from the battery to the controller, fused or with circuit breaker at the battery. You need to tap into the brake pedal wiring to get a hot signal when the pedal is depressed and you need to run the wiring to the rear of the van (underneath is easier than though the van’s interior). You need to mount the controller where you can reach it while driving and where it does not interfere with air bag deployment. I mounted mine below the little shelf that is below the USB/12VDC receptacles. The wiring from the controller to the battery and to the trailer connection (unless run inside) needs to go through the firewall.
    
5. 7 pin trailer connector.  The brake controller hot line, trailer lights, ground and optionally a charge line all come to the 7 pin.  The above referenced harnesses for the trailer lights have a four-way flat so I got a 7 pin receptacle that has a 4 wire flat pigtail where you plug in the 4 wire plug for the trailer’s lights.  I ran the 4 wire flat plug though a gourmet in a pre-existing hole to the van’s underside to the 7 pin and sealed it up well. The other connections have to be made with butt connectors or the like,  and properly sealed.  All wiring must be proper gauge and rating for the environment (esp when in the engine compartment).
    
6. A weight distributing hitch (WDH) and / or sway control might be needed.  My guess is they won’t be needed but if they are, then that’s a lot more to talk about.  In older Sienna’s the soft rear suspension was noted and some people add Air-Lift air bags to the springs.  Some argue that air bags don’t help the handling when the trailer’s tongue weight is on the heavy side, whereas a WDH does help (by keeping the front drive wheels down).  Like I said.. WDH, air bags and sway control discussion could fill a book and probably won’t be needed anyway but check you manual.

Now this is just a very brief overview and I left a lot of information out.  I assume you are hiring someone to do all this but if you DIY then I can offer a lot more help and suggestions. There are also a lot of helpful videos on etrailer.com but as far as I know, there is not a single video that covers everthing you need to know for your needs. I doubt that a Toyota dealer will help with any of this - they do not make the parts needed for towing and they do not support after-market parts. A good U-haul dealer or trailer shop should be able to do all this, however, if possible, I would find a guy who has added the trailer lights harness and brake controller on a Sienna specifically.

Another option is to get a vehicle that is built to tow and has all the pre-wiring in place.