I assume that you are doing food delivery as a contractor (https://postmates.com/). By "contractor", I mean that you are not an employee. As an employee, you would file a W-4 with your employer to tell your employer your Social Security number and the amount of withholding of federal income taxes. As a contractor, you probably gave the "employer" a W-9, which indicated only your name and Social Security number, because YOU are responsible for the federal taxes (income, Social Security, and Medicare). Instead of a W-2 at the end of the year, a contractor receives a 1099 to add to his/her tax return.
So if you have determined that you are a contractor (you can always call and ask Postmates), then you will have to file a Schedule C on your tax return (in TurboTax, this is the Self-employed version or Home & Business). On Schedule C, you will be able to show your income as well as your expenses. TurboTax will also add Schedule SE which calculates your self-employment tax (i.e., Social Security and Medicare).
Note that both of these forms are filed as part of your 1040 tax package; TurboTax will generate them automagically based on the data you provide in the "business" part of the interview.
It is beyond the scope of a simple post to tell you all the things you need to know to complete a tax return as self-employed. Note that Uber and Lyft drivers have exactly the same issue, and that those companies publish guide sheets to help their contractors complete their tax return. Ask Postmates if they provide similar documentation.
Also go find a book (like from the "Dummies" series or the "Idiot's Guide to" series - both series are quite helpful and do "dumb it down" for you) describing how to report your self-employment income on your tax return. It is worthwhile educating yourself on this issue.
There are two other things of note: (1) as a self-employed person, you are responsible for paying 15.3% of your net business income as self-employment taxes. This catches most first time Schedule C filers totally by surprise, because it means that they will owe much more in tax than they expected, and (2) you will probably have to pay estimated taxes during the year on a quarterly basis (form 1099-ES). As an employee, your employer takes care of both of these things, but as a contractor, you have to take care of them.
Also, if after all this, you tell me that you are actually an employee of Postmates, then forget everything I said above. You will get a W-2 in this case and enter that as part of your tax return. Note that due to recent tax laws changes, employee business expenses are no longer deductible, so forget about these expenses helping you out...but I bet that you are a contractor...
