Ooooh, you'd better sit down because this is going to be a long post (sorry!)
First of all you will need to get a water test kit because your fish has been put in an uncycled (unprepared) tank. It's not the end of the world but you'll have to put some extra work in to stop the fish getting sick. Have a read of this and it will explain about the nitrogen cycle. You'll need to do lots of water changes to keep the ammonia and nitrIte levels down.
Another thing to take into account is that goldfish get very large. Admiral Bubbles (best goldfish name ever btw) will be at the cute baby stage now but they get much bigger and they do it quickly. This covers goldfish growth, tank size and life expectancy.
A pinch of food once a day is probably enough. Goldfish are brilliant liars and will try to convince you that they are perpetually starving- don't fall for it! Give him what he can eat in 2-3 minutes and scoop the rest out. It might take a while to figure out the exact amount so go slowly to start with.
Cleaning is relatively simple, you'll need a bucket or 5 (depending on how big the tank is and how many trips you want to make). Syphon out around 25-30% of the water and use a gravel vac to clean the substrate. Unplug the filter and give the sponge a quick swish around in a bucket of water that you've taken from the tank (chuck the water on your roses, it'll do them good). Never, ever use tap water to clean the filter sponge, the chlorine will kill all the good bacteria. Then refill the buckets from the tap and add a bit of boiled water from the kettle to bring up the temperature so it's the same as that of the water in the tank. DO NOT FORGET TO DECHLORINATE!! Put the sponge back in the filter and switch it back on. Then put your feet up 
Hopefully that's a start, let me know if you want to know anything else. :)