How do i decifer which amount goes into which box from a paystub

You can't.  That's why you have to wait to get the W2.   The W2 might not match your paystubs. There can be several pretax deductions that affect it.

And don't file early! Last year (for 2018) the IRS put out some late changes and some early filers got tax due notices after they got their refund. Also if you file in an early batch they get stuck in processing and might even take longer to get your refund.

And You should wait because you might get a late W2 or 1099 in the mail. They have until Jan 31 to send you the tax forms. Or they may send you a corrected W2 or 1099 even later.