A few unrelated language points first:
There is no such word as 'tmrw'. This is a meaningless sequence of letters.
You can't say 'go to office'. 'Office' is a singular countable noun, so it needs an article.
Now to your question:
1. We mostly use the 'will' form when you are making a promise/offer, or making a decision at that moment. For example, in a phone call the night before from your boss:
'We need someone to be at the office to meet the delegates tomorrow morning. They're arriving at 9.30, but I can't be there.'
'OK. I'll go to the office around 9 o'clock tomorrow morning.'
2. We use the future continuous to say what will be happening, or is scheduled to be happening, a particular time in the future. For example, your sister might say to you, at some point after the above phone call:
'Can you give me a lift into town tomorrow?'
'Well, I'll be going to the office around 9 o'clock tomorrow morning. You can come with me then if you want.'