I am not sure, you are single or with family.
Acknowledging offer letter , you can simply write back to your prospective employer thanking them for the offer letter. But these are questions that should have come up in interview itself, which will give better insight on what is in store and a line would have been mentioned in the offer letter or covering letter pertaining to this.
Else you need to ask the prospective company politely that since you plan to join the new company on a particular date and since you need to book tickets, would like to know if the company would make t book or you need to make your arrangements.
This will make you know the company's position in regard to your travel and relocation expenses, if any.
20th December 2017 From India, Visakhapatnam
As previous member has said, you could have initiated your requirement right at the time of interview itself and told HR to include it when the offer letter was issued to you.
Today morning, I have given reply to the similar query. Click the following link to refer my reply:
20th December 2017 From India, Bangalore