I just started a new job and enrolled in a 401k Walgreens retirement plan how does that get claimed on my tax return? And how does it effect it?

The 401k will be on your W-2, so you will simply enter your W-2 exactly as it appears.  You do not enter the 401k anywhere else---just when you enter the boxes from your W-2.

You might end up getting a bit of Savers credit which will reduce any tax liability you have.