It depends on the day, but I normally spend about 20 dollars per day. It costs me 3 dollars each way to take public transit to and from work (so 6 dollars), I make all of my meals at home (so about 12 dollars), and I buy two 1-dollar coffees per day as a treat (so 2 dollars).
On an average weekday, I probably spend around 5 dollars (for fast food, or a drink from the vending machine at work.) However, on the weekends, I probably spend about $20 a day, on things like gas, groceries, alcohol, etc.
One of the easiest ways is to write down every penny for a month. Carry a note card in your wallet or a small notebook in your backpack and write down everything you spend. No guilt, no shame. You are just trying to collect information. Then you'll be able to tailor your spending to what matters to You. Good Luck.