The student hostel accommodation expenses in Singapore is about S$ 500 per month. Other living expenses is about S$ 500 per month. You might want to roughly calculate base on that and exchange proper amount of Singapore dollars before you arrive here.
You can get your Mobil pre-paid card in any terminal building of Changi Airport, or you can also get it from 7-11 stores, Cheers convenience stores, Singpost. The main telecommunications companies in Singapore are Sintel, Starhub and M1.