The first imperative (Пойдём) refers to the second person singular (ты). We use it when we address a friend, a child etc.
For example,
(Давай) пойдем в кино? (Let's go to the cinema)
Давай приготовим что-нибудь покушать? (Let's cook something to eat)
The second one is a more polite form which we normally use in a more formal situation, e.g. when you talk to someone you don't know or when you talk to an elderly person. So, we say
Давайте пойдем в кино or Пойдемте.
We also use this form when you talk to a few people, e.g.
Входите, пожалуйста (Come in, please). .
Помогите мне, пожалуйста. (Help me, please)
As you can see from the example sentences above, you simply add -те:
Пойдем - пойдемте
Входи - входите
Давай - давайте
Помоги - помогите
Hope that helps.