When do I use we and us as pronoun?

In traditional grammar, one uses 'I' when it's a single person (when it's just you), and 'we' when there's more than one person (you and a friend, you and your parents, etc.)

In modern days, "we" can be used to describe one person. An example is a consulting firm with no employees beyond the founder, a blog written by one person, etc. In these cases, the acknowledged existence of a single person would diminish the entity's authority so an improper use of pronouns is used to add weight to the entity's position.