What amount from box 12 do I use for my taxes?

You should enter all of them as part of your W-2.  

Some are informational only, but others are transferred to a specific form that will be included in your return.  For example, the information reported in box 12 with code W shows that there was a contribution made to a Health Savings Account (HSA).  This will require further information from you as you go through your return.

TurboTax will take any information that is reported elsewhere in your return and do it for you.  You will not have to decide where this information goes.

To learn more about the codes and what they mean, take a look at the following TurboTax help article:

What do the letter codes in box 12 of my W-2 mean?