To sum the numbers in a for loop statement, you will first want to set your initial variable to something before the loop. Since this is regarding addition, you will set the initial variable equal to zero.
Ex. theSum = 0
Next, in the for loop statement, you will want to take that variable equal to itself plus another variable. When you set it equal to itself, it will accumulate the sum and add the new variable as well.
Ex. theSum = theSum + newVariable.
Let me know if this makes sense! I hope it can help.