Charitable donations are only entered on a federal tax return as an itemized deduction on Schedule A.  The Total of All itemized deductions reported on Schedule A Must be greater than the Standard Deduction for your filing status to have any tax benefit.

Standard deductions for 2019

• Single - $12,200 add $1,650 if age 65 or older
• Married Filing Separately - $12,200 add $1,300 if age 65 or older
• Married Filing Jointly - $24,400 add $1,300 for each spouse age 65 or older
• Head of Household - $18,350 add $1,650 if age 65 or older

 I dont understand what that means, how much I would get back if i claim 12500$ in my standand deduction ( lets say if i donated total of 12500$

If you are Single then your Standard Deduction is $12,200 in 2019.  If you made a donation to a qualified charitable organization of $12,200 and that is the Only itemized deduction on Schedule A then you would receive no tax benefit for the donation.

If you made a donation of more than $12,200 then you would receive a tax benefit since your total itemized deductions are greater than your standard deduction.  

This assumes that your donation is less than 20% of your Adjusted Gross Income.  A $12,200 donation would not be limited if your AGI is less than $61,000

You might not get anything "back" for donating to a charity.  As simply as I can explain it--since you seem not to understand what a standard deduction means -- Your standard deduction (which is based on your filing status) is the amount of income you receive that is not taxed.  If you have other things to deduct from your income, like mortgage interest, property taxes, medical expenses, charitable donations, etc., then you enter those things and if they add up to MORE than your standard deduction, you might get a tax refund---IF you paid more tax than the amount of the itemized deductions.

Try the taxcaster tool:

https://turbotax.intuit.com/tax-tools/calculators/taxcaster/

STANDARD DEDUCTION

Many taxpayers are surprised because their itemized deductions are not having the same effect as they did on past tax returns.  The new higher standard deduction and the elimination of certain deductions, as well as the cap on state and local taxes have had a major impact since the new tax laws went into effect for 2018 and beyond.

Your itemized deductions have to be more than your standard deduction before you will see a change in your tax owed or tax refund.  The deductions you enter do not necessarily count “dollar for dollar;” many of them are subject to meeting  tough thresholds—medical expenses, for example, must meet a threshold that is pretty hard to reach.  The software program uses all the IRS rules that apply to the expenses you enter, and it tells you if you have enough to use your itemized deductions or if using the standard deduction is more advantageous for you.  Under the new tax laws, some deductions have been capped—there is a $10,000 limit to the itemized deductions for state, local, property and sales taxes.

Your standard deduction lowers your taxable income.  It is not a refund 

2019 Standard Deduction Amounts

Single                                              $12,200   (+ $1650 65 or older)

Married Filing Separately            $12,200   (+ $1300 if 65 or older)

Married Filing Jointly                    $24,400   (+ $1300 for each spouse 65 or older)

Head of Household                       $18,350  (+ $1650 for 65 or older)