Buy in blocks from the supermarket. Lightly salted butter is used for general cooking, and unsalted butter for baking.
You can store butter (opened or unopened) up to one month in a refrigerator. Opened butter should always be put in a separate refrigerator compartment or in a covered butter dish to prevent them from absorbing other food odours.
Tips & Tricks:
Butter contains 4 grams of fat per teaspoon. Enjoy it in moderation and try to limit intake by spreading it soft - so it goes further. Cool your toast before spreading butter on it. Use olive oil over cooked veggies instead of butter and avoid any bread and butter at night.
A serve of butter is calculated per teaspoon.
For baking, nothing really beats butter - use sparingly as a treat.