The pet shop or breeder will normally give you a container to take your gerbil home in. This is usually a cardboard box with airholes but gerbils have a habit of chewing their way out of these and so they are not ideal - particularly if you have a long journey home.
Therefore it is best to go prepared and take a more suitable container with you. This can either be a small plastic carrying box designed for the transportation of small animals sold in pet shops or any suitable sized plastic tub (such as an ice cream tub) punched with holes.
If you already have a cage and it is easy enough to transport then you can take this with you to go and collect your gerbil. However, do not fix a water bottle to the cage as the motion of the car, walking, etc will cause the bottle to drip and soak the cage. If you have a long journey provide the gerbil instead with a piece of cucumber from which it will gain some moisture during the journey.
Once you arrive home, place the gerbil immediately in the prepared cage and leave it to settle. It will take the gerbil a couple of days to get used to all the new smells and sounds of its new home and will find this time a little stressful. Therefore although you may be tempted to get your gerbil out and play with it, try to resist for the first couple of days so that it can first of all get used to its new surroundings and feel comfortable. This will help to keep its stress to a minimum.
After a couple of days the gerbil will become used to its new home and will generally seem more settled. This is the time when you can start to introduce your gerbil to handling.