How can I use welcome?

Excuse me, I'm a bit confused about the usage of the word "welcome." Can you please clarify for me how it can be used effectively in various contexts? For instance, how would one use it to greet a new customer at a store, or to respond to a thank you from a friend? Are there any common mistakes to avoid when using "welcome" that I should be aware of? I'm eager to learn more about this versatile word and how it can be incorporated into my daily communication.