Choosing this in the first step is always a bit tough. How much budget should I allocate for one day of advertising? How much should I spend? To what end? There is no simple answer to these questions. It depends on the industry you are in, on your competition, on your objectives etc.
Don’t sweat it too much though!
You can always change the daily budget as you are moving forward with your campaign. If you are a small business just starting out, you might want to test a bit, using smaller amounts during the first days. Start with 5-10$ / day and see how this will go. Remember that you are paying only when a user clicks your ad.
When your daily budget runs out, your campaign pauses until the start of the next day. If you notice your budget running out fast during the day, it means that there is some interest in your ads and that you might be losing customers later in the day. So, consider raising it to be competitive throughout the entire day.
Managing to spend a little less than your daily budget is usually a good sign.