Just stick him into filtered water in a small container and it will be fine. Also, when you go to the pet store, you will need to get a big tank. Goldfish grow up to a foot or more and live for many years. Goldfish in aquarium grow to 6-8 inches( for the most part, although some may grow to a foot, but I have yet to see any foot long goldfish that have been 100% grow in aquariums), and need at least a 20 gallon tank. A good rule is one goldfish per 20 gallons, then 10 gallons a goldfish after that. So you would need a 20 gallon tank for your goldfish when it grows full size. If you can not get it the a 20 gallon tank, a ten gallon tank will do until it grows past the 3-4 inch mark. After that, a 20 gallon is a must( you will need a 30+ gallon if it grows past 8 inches). What ever you do, do not get a tank smaller than 10 gallons for a goldfish. Also, you might here of the one inch of fish per gallon rule. This rule is used a lot by pet store employs. Do not listen to it. A ten inch fish could not go in a ten gallon tank. However, 20 neon tetras could go in a ten gallon tank. It all depends on the fish. Last, you will need to cycle your tank. Seeing that you already have the fish, pick up a bacterial supplement at the pet store. This will help keep your fish alive during the cycling process.
Hope this helps!!!