Need help! 5 days stay in SG

Accommodation in Singapore is expensive and will be your major cost. The cheapest hotels are around S$60/night (look for chains like Hotel 81, Fragrance Hotel, etc. and avoid the Geylang area, as that's mostly for prostitution). A more acceptable room costs S$100/night and up.
Alternatively, you could get a dormitory bed (in case you don't need privacy) for about half this price in a backpacker hostel. I heard good things about Sleepy Sam in Bussorah St., but have not used it myself.

Other expenses are food (S$15-20/day if you only eat at hawker stalls, double that for one proper restaurant meal per day), travel (S$5-15/day on public transport, S$50-100 if you do the same on taxis) and of course things like entrance fees, shopping, etc., which I cannot foresee here.

Have a great time!