I have 3 colums. In the first one I have a number, in the second column a have "yes" and in the third column I want it to SUM the number in the first column and a number in the second column associated with the word "yes".

Is it possible?

For example the sum with 7(first column) with 5 (second column associated to "yes") which will give 12 in the third column.

Completing Zohaib's answer, I would suggest
D1 - =IF(C1="Yes",A1+B1,A1)

assuming that if we have no "YES" then we just want to copy A1 over.

Also, if you want to keep the current file data, with the first value in column A, then the "YES" in column B, then
in F1 type "5"
in the Formulas menu / Define name set the name "Tax" for cell F1
in C1 enter