When you have got everything you need in the supermarket, you then go to the checkout counter and wait in line. If you have fewer than 12 items you can usually go in the express line. If you try and go in this line with more than 12 items, people can get cross, or ask you to another line. If someone is in front of you in the express line and they have more than 12 items, it is usually best to ignore this as it is the responsibility of the checkout operator to point out their mistake to them. If you have more than 12 items, go to one of the general checkout lanes.
In most supermarkets, they have on the counter a plastic ruler-like object that you can place on the counter between your groceries and the person's ahead of you. This stops your groceries getting muddled up with theirs.
If you are going to pay by cheque make sure that you have some identification to prove that it is your cheque. If the grocery bill is more than you can afford, you can ask the checkout operator to take some items off your purchase. If people do this they usually need to take off things such as cleaners that they can manage without for another week, rather than food items that they need.
Some supermarkets will pack your groceries into a bag for you, but others like Pac'n Save expect you to do it yourself. If it this type of supermarket, push your trolley to the section past the checkout where there are plastic bags/boxes and pack your groceries there.
Then take your trolley to the car, or outside to wait for a taxi if you have ordered one. Most people put the trolley back into the trolley line when they have finished using it.