You have to tip 20% of the bill minimum. You can round-up only from there.
It's considered very rude to tip less than 20% because in the US, most service workers are legally allowed to be paid less than the minimum wage ($2 or $3 per hour is not uncommon).
You should give $1 to a bartender for every drink you order. If it's an expensive city, you should give $2 per drink.