I have a 401K that I put money into every pay period and my employer puts into it as well. Where do I put the number thats in 12a when I'm filing my taxes?

Enter amounts shown in box 12 of your W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax.  Your employer has already excluded the deductible amount from the amount in box 1 of your W-2 so you don't get to deduct it again.  Employer contributions are not to be reported anywhere on your tax return.

You don't enter it anywhere except your W-2.  Your box 1 taxable wages are already reduced by the amount you put in the 401k, that's how you get your taxes reduced.  You don't also get a separate tax deduction.