Where in the TurboTax form can I deduct my computer usage/ home office materials?

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If you are a W-2 employee, you cannot deduct those employee business expenses any longer that used to be itemized deductions subject to reduction of 2% AGI.  They ended after being used the last time on the 2017 returns and were no longer available beginning on 2018 returns. 

If you are self-employed, you would use the Office in Home Form 8829 to take those deductions for use of your home.  The computer usage would be depreciation on the Sch C.