Furture Tense

We use I'll (= I will) when we decide to do something at the time of speaking:
-- Oh, I've left the door open. I'll go and shut it.
-- "What would you like to drink?" "I'll have an orange juice, please."
-- "Did you phone Lucy?" "Oh no, I forgot. I'll phone her now."

Shall I ... ? Shall we..?
Shall is used mostly in the questions shall I ...? / shall we...?
We use Shall I ...? / Shall we ...? to ask somebody's opinion (especially in offers or suggestions):
-- Shall I open the window? (= Do you want me to open the window?)
-- I've got no money. What shall i do? (= What do you suggest?)
-- "Shall we go?" "Just a minute. I'm not ready yet."
-- Where shall we go this evening?

I hope these will help