If you are pointing at an item you can see
- For things near you: これを [number + counter] ください。
- For things closer to the other person (like things behind the counter): それを [number + counter] ください。
- For things far from both you and the shop assistant (like when pointing at a picture high above): あれを [number + counter] ください。
For explanation of これ、それ、あれ, watch this video.
ください means “(please) give me.”
If you can say the name of the item you are buying
[item name]を [number+counter] ください。
If you are ordering multiple kinds of items.
[item name 1]を [number+counter] と [item name 2]を [number+counter] ください。（if more than 2 kinds, repeat the red part)
If you want to know the price:
いくらですか。How much is it?
ｘｘｘは いくらですか。 How much is xxx?
ぜんぶで いくらですか。 How much is it altogether?
ぜんぶ means all, whole, entire and by adding the particle で, it means “altogether” here.