You can use 'will' when you want to talk about something you have decided spontaneously to do or for a prediction which has no evidence. 'I think I'll go to the pub after this' or 'I think she will have a boy' (she is pregnant) 'Going to' indicates a plan (something you decided before you spoke) or a prediction with some sort of evidence/reason. 'I am going to go to work early tomorrow because I have a lot to do' or 'I think it is going to rain' (there are black clouds) Espero que esto esta claro...