Difference between Will and Be going to

Following site has a lot of sources to make them clear regarding tenses in english.

http://www.englishpage.com/verbpage/verbtenseintro.html

Will is used:

1. Lack of certainty when predicting something - possibility. E.g. I think it will rain tonight. (The speaker is not sure whether or not it will rain...it's just a possibility)
2. Spontaneous Decisions / Offer - as an immediate reply to something. E.g. A: 'I need to go to the airport tomorrow morning, but my dad can't take me.'
B: 'Don't worry. I' ll take you myself.' (Be used 'will' as an immediate response to what A said. It is also an offer - B offers to take A to the airport)
3. Threat. E.g. If you kiss my boyfriend I'll kill you. (The last part of the sentence - I'll kill you - is the threat)
4. Conditions / Real possibilities. E.g. If it rains I will stay home. (First conditional - used to express a real possibility)

Going to is used:

1. To express certainty. E.g. It is going to rain. Look at all those dark clouds in the sky. (The dark clouds are proof that it is definitely going to rain. When compared to No. 1 for 'will' it is obvious that the possibility is much bigger)
2. To speak of something which has been planned in advance. E.g. A: Dad, my bicycle needs to be repaired.
B: Yes I know, I've seen it and I am going to repair it later on today. (B has already planned to repair the bicycle) 

Of course this depends on the level of the class you are teaching, for example, I wouldn't teach WILL for THREAT or CONDITION to a pre-intermediate class.

The main differences are:

To express a PREDICTION: Will or Be going to
I think it will rain tomorrow OR
I think it's going to rain tomorrow.

To express a PRIOR PLAN: only use BE going to
(This means that you made plans in the past to do the activity)
A: Why did you buy that paint?
B: I'm going to paint my bedroom tomorrow.
(I decided to paint my bedroom yesterday and I intend on painting it tomorrow)

To express WILLINGNESS: only use Will
(The Phone is ringing)
I'll get it! (means that you are volunteering to do it)