Oil and filter change is basic maintenance task but as the OP said, if you have not done it before, let somebody else do it while you watch. The drain plug is 14mm. The next time, you will be ready to do it. Still, be careful as you might end up with a big puddle of oil in your garage. My personal preference - Mobil 1 10W-30 synthetic oil, OEM/Mobil Oil Filter and crush washer for the drain plug.
You can buy parts from IoS (Infiniti of Scottsdale), one of NICO sponsors. At 25% off MSRP, very hard to beat.
http://www.infinitipartsusa.com
If the car had just been serviced, how come the oil is dirty? What kind of service did they do? At 59K, you basic warranty (4 years/60,000 miles) is about to expire or is already expired. Your Powertrain warranty is still good (6 years/70,000 miles).
In addition to oil change, you also need to replace the fluids - transmission oil (Matic-J or Matic-S, differential oil (GL-5 80W-90), power steering fluid (Dexron III or better), coolant (50-50 with distilled water). At 59K, your drive belts and spark plugs are due for inspection/replacement. The throttle body and MAF sensor need cleaning.
How's the engine air filter? If the A/C air doesn't smell good, the cabin air filter needs to be changed. Have you done a high-speed braking lately (>40K MPH)? If you felt vibration, the rotors need to be turned/replaced, together with brake pads.
We can go on and on about the things that need to be done with your car. But before you attempt DIY activities, makes sure you have the right tools for the job.