If you are a W-2 employee then none of your W-2 related expenses are deductible on the federal return. Period.
If you are a self-employed musician that reports business income/expenses on SCH C, then your instrument purchase is a business asset. It gets entered in the Business assets section, classified as equipment and depreciated over 7 years. In TurboTax you'd select Tools, Equipment, Machinery, Furniture, then General Purpose Tools, Machinery & Equipment.
If the program offers you the SEC 179 deduction and/or the special depreciation allowance, then take it *ONLY* if your business as the *taxable* income from which to claim it against. (Otherwise, it's a waste of your time and will make no difference in your tax liability.)