Firstly, you got misled on the tips. Always tip 15%; 20% if the service was especially good. 5-10% is going to make them spit in your food.
Let me help you out a bit. Those are the prices to buy USD against SGD - they're the number of SGD you need to pay to buy one USD.
So when you're buying USD, you want the lowest price (1.2395). And when you're selling USD, you'll want the highest price.
(That all said, you don't need or want to take too much cash with you. I usually only took $100 or $200 with me - basically enough to get a taxi to the hotel when I arrived - and used credit cards for everything else. Carrying thousands of dollars in cash is just asking to get mugged.)
Actually credit card is no no especially big item as service charge now a day is anywhere in 3 to 3.5% plus very bad exchange rate.

