I'd like to start reselling clothes on Poshmark for profit -- Is there a minimum profit I need to reach before I report to the IRS, and how do I report that income?

[TomYoung]

"Is there a minimum profit I need to reach before I report to the IRS"

No.  Generally as a taxpayer you need to report all income from all sources. 

This kind of question comes up all the time with respect to interest income since payers of interest aren't required to generate a 1099-MISC for amounts less than $10 but that doesn't mean you don't have to report your $2.51 interest income.  You are obliged to do so.

As far as reporting this activity to the IRS, that's done on Schedule C which is incorporated into your income tax return.  The profit will be taxed at "ordinary" tax rates, (like wages, interest, etc.), and is also subject to Self Employment taxes, (Medicare and Social Security), at roughly a 15% rate. 

TurboTax supports the preparation of Schedule C and will calculate any SE Taxes for you, but you will need to use the TurboTax "Self-employed" version if you want to prepare your income taxes online.  In the desktop versions all of them support the preparation of Schedule C though the version with the most "focus" on self-employment is the TurboTax "Home and Business" version.

[Carl]

There's no such thing as a "minimum profit". In fact, you don't have to make any profit at all really. If you are just "open for business" for even one single day during the tax year, then you may (or may not) be required to file a tax return. Even if the  business has a loss. It is very common for a business to not make a profit in it's first three years too. But that doesn't mean you're not required to report it.

You are required to file a tax return if certain conditions are met.

 - You have more than $12K of earned income (such as that reported to you on a W-2 as one example) that would include self-employment income.

 - You have more than $400 of self-employment income.

 - You have more than $1,050 of investment income

So if you have $11,999 of earned income reported to you on a W-2 along with $399 of self-employment income, that puts your total income over $12K and you are required to file a tax return.

Note I am assuming you would be filing a single.